If you want to add named colours to the army painter then take a look at the UberPainter mod. There's a file in there (something like army_painter.lua or similar) that specifies the colours in the colour list. It's just a text file so you can open it in Notepad or anything similar.
If you want to add new colour options (so additional selections beyond the existing Primary, Secondary, Trim...) then you need to do some major hacking of the underlying game by modifying one or more DLL files by editing them in a hex-editor. It's definitely not recommended, would be a huge task, would be completely incompatible with any other version, and may not even be possible.
If you want to add painted areas to models in the army painter then you need to read the
Skins@HWT Tutorials and get a copy of the
DoW Texture Tool
If you need any more help then you'll need to be a bit more specific.
Also, now I've noticed your subject on the post (I normally skip them as they're fairly consistent) I'll break this out into a new thread, since it has little/nothing to do with the
original topic.