As an untalented artist (!) I sometimes have a need for changing colours in (for example) a .png file. I know the RGB values I want, but I don’t have something I am familiar with that can allow a colour to be chosen that way! I still don’t get HSV or the other alternatives - and some otherwise usable progs have preset palettes…
Hwat do I need to add? (apart from talent, of course)
If you want to replace one colour with another, imagemagick is a perfect CLI tool for that.
For example, this command will replace red pixels rgb(255,0,0) with lime green pixels rgb(0,255,0) in the INFILE and save it as OUTFILE:
convert INFILE -fuzz 0% -fill 'rgb(0,255,0)' -opaque 'rgb(255,0,0)' OUTFILE
The -fuzz parameter changes the sensitivity of the source colour detection. Increase the value to make it less specific.
If you want something more interactive, Krita and GIMP are probably the best tools for that.
Pinta, KolourPaint, and Gpaint are very simple, but pretty decent image editors (similar to, but much better than Paintbrush).
Also, check out this great list of raster graphics editors:
https://wiki.archlinux.org/index.php/List_of_applications#Raster_graphics_editors