Here's a Mason Jar brush,
that will paint in any selected color.

There are a few tricks I employed,
which may be applied to this brush to get the most favorable results.

There are a few layers that need to be added.
First paint with a single click,
onto a transparent layer of your canvas with the brush.
Using your color of choice.
In my screenshot below.
I use an Orange Hue for my jar.
This is the first layer named "normal".



Next using the Fuzzy Select Tool .
Click the outer portion of the canvas.
Creating a selection on the outside of the jar.

Now add a new layer for the white background.
Invert the selection,
(select>invert) and fill this inverted selection with white.
To create the background layer.
I named this layer "white bg" and moved it to the bottom.

Next make a duplication of your colored jar layer and set the layer mode to Multiply.
This layer is named "Multiply".

Duplicate this layer and lower it's opacity to about 80,
or until you're happy with the color.

Last step,
is to go back to the white background layer and feather it's edges.
Use the fuzzy selection tool to create a selection,
on the out side of the white background.
Once this selection is made.
Go to Select>Feather and feather the selection by 5.
While this selection is still active.
Go to Edit>Clear.
To clear and feather the edges of the white background.

You may need to repeat the feathering again,
by 2 or 3.

When complete.
Your jar should look something like the image below.






Feel free to download this brush and use it as you wish.
Free to use, no applicable usage restrictions apply.

Great, glad to hear that.