Good topic

Perhaps, apart from the film grain filter which basically adds noise like the HSV noise filter.
I used the HSV noise filter on copy in overlay mode and played with the mask to take the shine off the face. Add a tint in color mode

looks good, wbool63

To post, reduced by more than 50% the size of the image.
For the details would not be lost with this reduction, featured you can see the before and after, a less reduced selection.

Used high pass filter, masks, and grain merge to this result.

I either read about this technique here or on Pat David's site. but the best way to make an image greyscale is to use the Colour>Map>Sample Colourise filter. If you greyscale an image using the desaturate filter or by changing the image mode to grescale and then check the number of unique colours in the image by going Colour>info>ColourCube Analysis you'll find that only 255 colours are used.

So download your image into gimp and create a new layer with a black to white gradient along the layer, using the colourcube tool as mentioned above you should have at least 1786 colours, if not then you layer may be to small and needs to be bigger, then select your image and run the sample colourise filter selecting your gradient layer as output source.





Once the filter has run you can again check your colour values, in this example I ended up with a greyscale image made up of 1970 unique colours.

Your right it could have been either, True PseudoGrey in GIMP or Gimp Chat True Pseudogrey in GIMP.

Just had a search and found 24 Public Domain Photos for you to play with.
should be in two separate attachments

Another

Tried to sharpen this one but couldn't get the blur out of it.
Sharpened and used texture enhance. may have tried deblur also.
This photo is in one of the attachments in the last couple of posts.

Thanks He4rty for a reminder about this pseudogrey. I installed the filter and played around with it. I can definitely see the difference when I go to pixel level between the two methods (simple desaturation and using the pseudogrey filter). At a normal viewing levels I can not see a difference but perhaps in some circumstances it may well be visible. I used Sallyanne's bride image.
Interestingly, if you place both methods in layers above each other at say 10,000 magnification (1.5 cm pixel sizes) in difference mode there is no difference noted. (no highlighted difference pixels). Yet individual color pick tool info window comparison on the two layers show a difference.
That may be due to the differences in colour maths (probably 8 bit wide) I really don't know. But my biggest problem is where the magic figure of 1768 is derived from.
More work to be done on luminosity masks with this filter to see if this extra colour gradation can assist in better duo or tritone images.

@Sallyanne, thank you for finding those images , I have downloaded the lot

just another trial (very quickly done)
one of the layers used maxRGB

another one
playing with pseudogrey, colorize, saturation, smooth a little (various layers)
still a quick go...

A gray color is one where the values for the the red, green, and blue components are all equal. A "pseudo-gray" color relaxes this requirement and allows the components to either be equal or differ by one; the idea being that such a small change in the resulting hue will not be very noticeable to the human eye (which is more sensitive to the intensity of light than the color).

So for a given gray value (where r=g=b), we can produce six additional "pseudo-gray" colors as follows:

Of course, the next color to follow this pseudo-gray sequence would be (r+1 g+1 b+1), but that is actually a "true gray" color and so starts the next sequence of seven pseudo-grays.

So for 256 gray colors having values of 0 to 255, there will be 1792 (7*256) pseudo-grays -- except that for the lightest gray (255 255 255, AKA white) it is not possible to increase the component values to generate the six pseudo-grays, thus only 1786 different pseudo-grays are possible.

playing with another of the SallyAnne's images

(retinex)

Seems sharpening helps adding some texture back.

I tried with shadow recovery, color enhanced, sharpening, contrast enhanced, edge enhanced

Thank You Saulgoode , I tried all sorts of little divisions by 8, by 7, by 255, by 256 . nothing made sense. But your explanation makes perfect sense.

You all are great masters of photography. I think I may learn something as you help giving relevant explanations. Thank you my friends.
I love the rose, wbool63. it's a beautiful transformation.
Dinasset, the final result of the bride photography is fantastic!
Here my try, Firstly I corrected the background by colour levels and corrected its horizontality. Then I applied desaturation to the woman layer and set the layer at grain merge.

dream of Japan!
you started playing with difficult tasks, Issabella, go on!

Thanks dinasset. You're very kind.