Gimp: change one colour to another?

Question

Simple to explain: possibly not so simple to do.

In Gimp, I have a green button GIF image: it shades from dark green to light green, against a transparent background.

I would like to change it to blue, and keep the shading, so it shades from dark blue to light blue.

Anyone know how I can do this? Can't find an explanation by Googling!

Thanks.

Answer 1

You're looking for the Color-Menu, and especially the function 'Colorize'.

Answer 2

I suggest rotating Hue in Colors → Hue-Saturation.

Given an initial image like this:

You change Hue first and then adjust Brightness and Constrast to your liking.

The result looks like this:

You can easily make it colder or warmer, darker or lighter, more or less saturated.

Answer 3

On simple small graphics, what I do is Zoom in a lot, then change the colors myself using the brush.

Just change the brush size to 1 pixel.

Answer 4

I don't know how to do it in gimp, but ImageMagick has functions to separate and combine color channels. See this page.

I was able to convert a green gif to blue with this command:

convert input.gif -separate -swap 1,2 -combine output.gif

I guess that 1 and 2 refer to green and blue channels, 0 is red.

Answer 5

The first successful method I found to do the same sort of thing was to use ImageMagick, but with a different operator than the earlier-suggested --separate/-swap.

The different operator is either -recolor or -color-matrix, depending on ImageMagick version.

Specifically, because I knew a "master color" of the original image (green, in your case and mine) and I knew the "master color" in the desired result image (blue, in your case, orange in mine), I gave ImageMagick' -recolor/-color-matrix option a transformation matrix with values that reflected the differences in each of the RGB channels.

For example, in my case, the fully green color was R:141 G:198 B:63 and I wanted it changed to R:231 G:159 B:70. The rest of the colors wanted to be changed equivalently.

So that means I wanted new pixels' R value to be 231/141ths of the current red value. Green values to be 159/198th of the current values. And blue values to be 63/70ths of the current values.

So:

convert input.png -recolor      "1.638297872 0 0    0 0.803030303 0   0 0 1.111111111" output.png

or

convert input.png -color-matrix "1.638297872 0 0    0 0.803030303 0   0 0 1.111111111" output.png

or

This seemed to work in a flash after fruitlessly spending a lot of time wrestling with Gimp/plugins and paint.net/plugins.