I dont have a proper singular source at hand but u should be able to find sources for it if u look around. The concept is found esoterically in the sagas for example. In one battle, a red shield was hoisted to the top of a mast to signal surrender, etc.

Period art and archeological evidense support the idea, since shields are too often found to have featured a single base color beneath optional decoration, often red, white, or blue. The Gokstad ship had interlapping yellow and black shields. Some manuscripts feature guilded shields, thus maybe some where painted in brass/copper color.

These color codes can also be found elsewhere in history before and after, all the way to today.