There are three ways of adding color - duplicate stitch, stranded (fair isle) and intarsia.
Duplicate stitch is done after the work is finished. It's done by adding color over the stitches.
Stranded (fair isle) is usually done in the round and used colors that go all the way around in some pattern.
Intarsia is done flat and is for larger blocks of color that is only in one area..like a Santa on a stocking for instance. You have a separate section of color for each color change. They are wound on bobbins, butterflies, or just a long tail.
I suggest practicing any method first to get the hang of it. There are videos under the video tab/advanced techniques.