When is it the right time to add additional color maps to your model? This is a ramp model I was using to practice with, it's UV mapped with 1 1024x1024. Is one image always easier on the GPU than multiple? or is one 512x512 with 2 other 128x128s for decals better?

1024x1024 is a resulution thing, so yes itll take a few thousandths of a milisecond longer than a smaller one, but it will look better. Additional color maps should be added if and only if the texture is as full as you can have it while maintaining good quality. Also the less images you have, the less likely you will hit the material error