colorize an area of (mainly) one color to a given target color in GIMP

Question

I have the following image:

I want to change the colors so that they match my own Xylophone. Since there are in-between colors at the borders, just selecting a color and filling the selected area with my target color is not viable because the in-between colors will stay unchanged. See here (zoomed in), original green color and blue-colored version:

So my preferred approach is to select a larger area that contains all the in-between colors and some black border and colorize that whole selection. That way the main color changes, the in-between colors change accordingly, and black stays black. There are a number of color changing options in GIMP like "color balance", "hue-chroma" or "colorize", and I tried all of them (i.e. the ones I could find in the Colors menu). But I could not find any one that allows me to specify a target color.

Here is my target color:

So I thought I could just type in my RGB values in one of these colorizing options, but I could not find any option that takes an RGB value as input. I tried taking the L-C-h values instead, assuming it is just a different way of specifying RGB values, and entered them in the option "hue-chroma":

But firstly the maximum Hue value in that option is 180, so I could not use my actual value of 281.5 and secondly, this does not look anything like I want it: I expect the black to stay black and only the colors to change. So I am obviously not using that coloring option as intended.

How can I change the color of a selected area so that

• a specified color (here green) turns into a target color (here blue)
• white, grey, black stay unchanged
• in-between colors change accordingly so that the newly colorized image is as smooth as the original ?

Note: I know that I could select the green area close to the black border, grow it a bit, feather it a bit, then fill the selection with my target color. That will also give smooth transitions. But since this depends a lot on exactly where the feathing starts and how far it goes, this is quite fiddly. I am looking for a more robust solution.

Answer 1

Using gimp

1. Make a selection with the "Free select tool" you can make it in the middle of the black line.

2. Play with the "Hue Croma" tool. Mainly change the hue, but you can also play with chroma.

As the black has no saturation it will not be affected.

You might be tempted to change lightness. If you need it, you need to refine your selection to leave the black out of the selection or select the black (or resulting gray) and re-adjust it.

It is the same solution unpropan mentioned, but using Gimp tools ;)

Answer 2

Get Krita - it's also free. It's HSV/HSL adjustment filter maybe does in Colorize mode what you expect:

It's applied to a selection which contains also some black as you did in your own attempt.

I do not have now GIMP. No idea if its colorize does the same.

Answer 3

Your image has a very low quality, with very strong JPEG compression artifacts (and the fact that it was later re-encoded with a better JPEG quality doesn't change anything to this), and these artifacts prevent a proper technique that handle anti-aliasing pixels (remove color by painting in Color Erase mode, re-paint in Behind mode) from working satisfactorily.

You can try Color > Map > Rotate colors to map a range of hues to another (but you can't change saturation/lightness directly).