Some notes on editing icons for the NWN style. Shouldn't need general/DM/Emote icons.

Existing Icons Info

NameIconRough Colour CodesNotes

Feat

Domain

7da8d1

White and black contrast

White fading into blue

Domains are similar although just a different more square design with a rounded black border and the background is two toned blue.

Spell

8b7486 (more drab) or 7c5574

To convert seems 3d2439 works well (61.4/35.8/57.2)

Similar contrast to feats

Purple-ish colouring

Spell Scroll

White version applied to the scroll iconThis is the spell icon but made into a white form. Black is transparent.
Skill


Green similar to Good Effects
Good Effect

2dca25 or 45/201.6/37.5 when converting other icons with colorizeProject hasn't made these transparent (yet)
Bad Effect

Pure red for converting is fineProject hasn't made these transparent (yet)
Class


Golden yellow

Colouring Icons

To edit a image to a certain colour first greyscale it (Colours → Desaturate → Mono Mixer).

Then a rough 60% opacity Burn use of the Fill tool. You set to foreground colour with the right colour option as per above. Tweak to fit.

Alternatively you can use the Colorize option although sometimes the blacks change excessively with using it.

Then add the border after removing bits with the magic wand. This may be easier done when black and white. Remember you can add areas to the existing selected ones to do two separately coloured but still relevant areas.

To get a version with a black alpha layer export with no background colours and set the background colour in the selector to black. https://www.gimp-forum.net/Thread-Change-transparent-color-to-white 

Spell Scrolls

Need to do this process not done yet.