What are the main colors?

It depends on whether you are talking light or paint/dye, the main colours are called the Primary Colours

Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three are often used; for additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green, and blue. For subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are cyan, magenta, and yellow (or blue, red and yellow)