[Gimp-user] Why is this color mapping not working?

[rich404 <forums gimpusers com>]

> HI, 
>
> I'm using Gimp 2.8.18 on Mac High Sierra.  I want to do what I hope is
> very simple -- switch out one color with another.  I wanted to change
> the white to a dark gray and teh black to a lighter gray.  I first
> changed the white color, which went ok.  Then, I want to change all
> the black to a gray color -- #D3D3D3.  I open my image, go to the
> Color menu, and select "Map" and then select "Color Exchange".   Then
> I select the relevant colors and click "OK".  However the result
> (attached) is disappointing.  It is very difficult to read the numbers
> in the scale.  Am I missing something?  Or there others colors at play
> here?
>
> Thanks for any advice, - Dave

Not the best way, guessing some sort of selection to choose the background
leaving semi-transparent pixels around the text.  A jpeg especially tends to
promote artefacts around text.

A better way.

Remove the background using Colour -> Colour-to-Alpha White is default 

Lock the transparent pixels on the layer using the alpha lock in the layers
dialogue.

Bucket fill the layer with the d3d3d3 gray.

Add a new layer under and fill with the darker gray.

Something like this is best exported as a png which because of the 2 colours is
smaller than a jpeg.

screenshot and xcf is attached. xcf should still have alpha lock in place as
example.

rich: www.gimp-forum.net

Attachments:
* http://www.gimpusers.com/system/attachments/818/original/no-selection.jpg
* http://www.gimpusers.com/system/attachments/819/original/scale.xcf

-- 
rich404 (via www.gimpusers.com/forums)