About Color Images

There are only two forms of color light in this world: additive and subtractive. In the additive world, a
light source is needed to create colors. When the primary colors (red, green, and blue) are combined,
they will create a white light, as shown in 3.0.

Additive color refers to the practice of combining non-reflected colored light. If
we take the visible light spectrum and divided into thirds, we would see that
red, green, and blue are the predominant colors. When these three colors are
added together in equal amounts it will make white light. In the same way, if
you add equal amounts of only two of these three colors you can create
secondary colors of cyan, yellow and magenta. By using different amounts of
each colored and strength, you can create all kinds of derivative colors.
Television and computer monitor are good example of this theory in action.
By uses thousands of red, green and blue pixels in combination with different
value of light, they can produce many different colors, from white to black and
all those in between. In the subtractive world, color is determined by the absorption
of light. When the secondary colors (cyan, magenta, and yellow) are combined, they
will create black-brown, as shown in 3.1.

Like a common ink jet printer, shows us a good illustration of this subtractive
at work. In creating inks for printing, impurities in the pigments result in muddy black-brown
when you combine cyan, magenta, and yellow together. To achieve rich shadows and pure
blacks, black was added to the printing process to be able to create those colors. By combing
additive primary colors togethers you will produce the subtractive primary colors, and if you
combine the subtractive primary colors together, you will create the additive primary colors.

Therefore, understanding these principles can be very useful when identifying and correcting color
problems in your images. A good example of using these principles, would be, trying to remove a color
cast. Let’s say you have an image that is too blue or has a bluish tint to it, because the flash wasn’t set
correctly. You would have two ways to approach this problem: one, you could either increase the
other 2 primary color to equal the overpowering blue tint, or decrease the blue to match the other 2
primary colors to correct the colorcast. In both cases, you will be able to remove the color cast
from the image. The biggest problem is finding that right settings to fix this problem. So, we could say
this, color casts are created because there is an imbalanced of 1 or 3 of the 3-primary color of light:
all three colors of light must have the same amount of volume to be able to produce white as
shown example 3.0.

In our next example 4.0, we have a photo that could have been a black and white
at one time, but has just faded over the years.

There are a couple different ways to return this back into a good black and
white image. For this illustration I’m going to show you how to use the levels option; but this
time where are going to adjust each of the 3-color channel to make each one equal to the other two, thus
restoring it back to a nice black and white looking image again. And here is what I did.
First, I change the color channel from Values to the red channel to work with, and I adjust the
gamma sliders according to the points indicated by the ranges of the red channel in the photo,
not adjusting the midtones range (Clamp Input). From the main tool bar of Gimp and go to.

Colors > Levels
Looking at the next image, you can see the settings I used. I want all 3 color channels to
have the same amount of color range and level range to restore the black and white image.

Once I made the changes I wanted, I Then changed the color channel to the green
and repeated the same steps, I want to slide my lightest point and darkest point to match my color
range for that channel. Once I made those changes I Then change the color channel to Blue and
repeat the same steps once more, by sliding my lightest point and darkest point to match my color
range for that channel. Then click OK

Now as you can see there is a slight tint to the image left from the Levels corrections,
This can vary from program-to-program functions of how they created the levels tool. In Gimp they
created a way to remove this (FX) in the process. To fix this...

Go to Main Tool Bar >Layer > New from Visible
Then Layer> (Shift + Shift + D) Duplicate Layer
Chane the top layer blending mode to HSL Color with the Opacity at 50%
Next go to, Colors > Invert
And then right click your layers palette and Merge Visible Layers > OK.
And you're all done.

So, by now you should have a better understanding how the additive color light
source works, and how each color channel values treat that light; the red, green, and
blue. In the computer world we have values that ranging from 0 to 255 instead
of percentages. Therefore, if each primary color has a value of 0%, no colored light
source, meaning; it has 0 red in it, or 0 green in it or 0 blue, you would end up
with black, there’s no light source. So, this means that white light is made of the 3
primary colors (RGB) with a set value of 255 per channel. As shown in previous example 3.0
to produce the white light in the middle by using these 3 primary colors.

A good way to determent values of a color cast is to use the Color Picker tool; a simple tool but very
powerful at that, it can collect the 3-color light values of any spot on an open image. By using the
Picker Tool, you can get the information you need to see and why it is not as white as you would like….

So now, if your image has a color cast for whatever reason(s), old image, poor scanning setting, a
camera setting or just faded. You can correct this in a few different ways in Gimp; one
method would be to see what the value of the 3 colors (RGB) channels are and how they relate to
each color channel. This way you can see what’s lacking in your image to make it possible to fix
the problem. In simple terms, which color channel(s) need to be adjusted and how.

Therefore, let’s say you took a sample reading of an area where the color cast is showing in your image,
a place that should appear to be white or very close of being white, and by doing so, this is what your
sample area revealed with the Color Picker Tool….

With this sample reading, in order to correct the color cast from your image,
you would subtract each color value from 255 to find the color value that is needed to
correct the overall white balance by correcting each of the RGB channels of light.

To correct the color cast, you would create a new layer above your image and set
the blending mode to HSL Color (Legacy) with the opacity set at 50%, and fill it with that color….
R:197, G:242, B:245 = The opposite color of the Color Cast And then merge it down.

Now there is a way to do the same thing as in option one, but in a
different way. This time there is no math calculations involved to find the
right color (the opposite one) needed to use, so to equal out each color channel (RGB).
Here we have a image with the camera setting was not set to the correct setting to take
a good picture. My bad!, So with example 5.2 here I will should how to fix it.

The first thing you need to do is,
Dulpicate (Shift + Crtl + D) the image in the layer palette
Then from the main tool bar go to > Colors > Invert
(to reverse the Image colors.)
Next using the Color Picker tool to change the foreground color, select an area from the
top layer, an area that should be white on background layer that is underneath the top
layer to properly fix the color cast. (Selected color should not be solid black).

Now Layer> New Layer >
Change the Blending mode to HSL COLOR, Opacity at 50%,
fill with foreground color and then click OK.
See next image for help. Where to select and what you layer palette should look like now.

Now for the Magic.
Delete the middle layer and select the top Layer
Goto > Layer > Merge Down

As I noted before, if your adjustment for the top layer is set at
50% and it don't fix the problem, you have not correctly selected the white
balance area and you are slightly off target. This doesn’t mean you can’t fix the white
balance with it. It’s just giving you clues about the sample color
you selected doesn’t make all 3-color channel equal in your image
and you'll have to try again.

In my example I used here, I selected my color sample under the
kitty nose and to the right, where the arrow is pointing in the example image,
not behind the kitty ear where it is washed out.

Now you're probably thinking, why go all through that to fix the image when
I have the Equalize Tool, which by the way can fix it with one click, what do you do when it
won't fix it. Then what. What do you do, how will you fix it, that's why you should learn how to do
this the old way and I'll show the reason for it in the follow example!

So here is the Problem Child Image

Looking at this image, the Equalize cannot fix it. Why, because it only knows
one way to process it, based on what it was programed to do. Like I said before, the Tool does a
good job on a lot images, but in this case it fails. But I will show you how it was done in the
early stages of photo editing. Here is Equalize version of it.

Looking at this image you can see that it focus on the white clouds and the rest is all
wacky looking, and that because it broke one of the rules I spoke about really, when using the Color Pick Tool.
Don't select the direct should to be white area but be very close to it. You'll have a better chance of getting it
right the first time. So here is my process.

1. Duplicate the image Layer
2. Invert the Color
3. Use the Color Picker Tool to get my color. (The X is my choice)

4. Created my new HSL Color layer and fill with my selected color at 50% opacity.
5. Delete the middle Layer and Merge down
6. Duplicated Layer change blending mode to screen.
7. Created a copy visible of image and pasted as my top layer
8. Change that layer blending mode to screen
9 Created Mask Layer - Black and painter the lady area with white in the mask.
10. Merge Down visibly
13. Last step Open the Hue-Chroma and made this changes. Hue -4, Chroma 4, Lightness 3.
All done with no big head acks.

Now after a few touchups and tweaks, this is what I have.
This image was probably a polaroid photo

Now about this process, you should note that this will only correct one
color cast in an image. If you have two different color cast, this will not remove both
at the same time. Which means you will have to do some serially coloring toning.
What creates a two-colored cast is because of the image is faded from time, and may also
have been taken with incorrect camera setting.

Hope this helps!