I mostly use acrylic paints. You can get them in the crafts department of Walmart & many other stores.
I use the smallest/finest point brushes I can find. Also available at Walmart for cheap (they usually have a great inexpensive 5 pack)
Exacto-knives for custom work. Just be careful not to cut yourself (as I have done several times).
This is the type of model glue and model sculpting clays that I use when customizing.
Finally when the custom is all done I spray with a finish.