Re: [Gimp-user] map all colors that aren't already white/gray/black, to #808080?

[Ofnuts <ofnuts gmx com>]

On 24/03/17 23:09, ottenm wrote:
> I think I got it all crammed into the subject line!  I have some images with
> color spots in them that I would like to turn gray, but I want all the color
> spots replaced with the same shade of gray (not to darker/lighter shades of gray
> depending on how dark/light the original color was).  Also want original whites,
> grays, and blacks left as is.
>
> I started using Color > Map > Color Exchange but the threshold is either missing
> too much or grabbing too much.  Not sure how to select everything that isn't
> white, gray, or black.
>
> Hope that makes sense, apologies if it doesn't or if there's an obvious fix I'm
> missing due to genetics or disposition.

Use the Color selection, but use the criterion selector  '"Select by", 
at the bottom to select by Saturation. Click on one of your greys and 
this will select only the pixels that have a saturation of 0 (or very 
close to, depending on threshold)(in other words, the grays). Then 
invert the selection and bucket fill with grey. Caveat: likely due to 
round-off errors, the saturation value on black ad very dark grays is a 
bit erratic and some pixels may not be selected in the initial selection.