Is called color holds and there's a lot ways, but I usually work with two: If the lineart is inked and "bitmaped" (coming from a black and white bitmap image, not a graycale)and very sharp, I just select and fill with color the lines. of course, for that you need to have the lines isolated in a layer and the transparent pixels blocked. The other way is create a layer over the lineart layer, filling parts with the color you want. Then make click between both layers while holding alt, and you'll "mask" the upper layer content with/in the layer below. I usually work this way when I'm colorin' directly over pencil works, no inks. Hope this helps!
What is the difference between anti-alias off and anti-alias on? And the flats colors are painted on one layer or you make differents layers for every color and then merge them?
You will see the difference between anti-alias off and anti-alias on in the selections borders... Anti-alias on looks soft and nice but is less effective to make the selections later. Anti-alias off gives a more "ugly" border, buts work very nice for selections. If the artwork is big enough, that "ugly" borders are not visible. The less layers, the best, IMO. I use one layer for the flats, and more just in the case the artwork include some transparency stuff like glasses, ice, etc. Hope this info help you!
That helped me ALOT! I'm using Photoshop CS3 right now so I think there's no anti-alias option, but I'm going to purchase CS5 and put what I learnt in practice. Thank you.
Also, great job!