@caycedo (tl;dr: jump to last paragraph) Heraldry has a limited color range. It uses two basic types of tinctures: colours and metals. (there are also stains and furs, but furs are special combinations of colour and metal.

The metals are or (gold or yellow) and argent (silver or white).

The colours are gules (red), vert (green, in Spanish sinople), azure (blue), sable (black) and purpura (purple).

One composition rule is that two colours can't touch each other and two metals can't touch each other. This rules apply to ordinary divisions, such as "terciated per fess" which is equivalent to a horizontal triband flag, or "terciated per pale" (vertical triband). It also apply for charges (figures) in a field (each basic division), for example a lion in colour can go on a metal but not on other colour. The rule does not apply to details within a charge or for divisions that have three fields converging in a point.

To simplify: light colors in the flag (white and yellow) don't touch each other and non-light colors (red, green, blue, black, purple) don't touch eachother.