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Hi Gimpsters
My problem is contrast between white foreground and white background.
The line between foreground is indistinct against the background for adding a Layer Mask. I need more contrast. Easily done Colors>Desaturate>Color to Grey. But after applying the Layer Mask how do I get my original colors back? Or, as usual, am I going about this all wrong?
I'm using 2.10 in Zorin.
Much thanks,
Bry

Two layers
On the top layer the layer mask should be just black (for transparency) and white (for opaque)

[attachment=10557]

You can get that using fuzzy select with a low (4 or 5) threshold value on the background. (or any other way you prefer)
The resulting selection creates the mask Layer -> Mask from selection (inverted)
Then adjust the top layer as required.

example: https://i.imgur.com/UOjWPhj.mp4

Thanks, Rich... let me give that a try and I'll let you know how I made out.
Bry

Rich gave you a solution but the important idea to remember is that the selection applies to the image, not to a specific layer.  So you can create a selection using a layer, and use that selection on some other item of the image (other layer, mask, channel...).

Hey Rich,
Just wanted to say thanks again for your advice. You did a lot of work and I appreciate it. The Fuzzy Select Tool method worked well and I was pleased to find I could also touch up the mask after, by opening a brush.
And a special hello to Ofnuts, thank you for your comments.
All best,
Bry