Every potion effect type has a certain fixed color, which can for example be found in the minecraft wiki: http://minecraft.gamepedia.com/Data_values#Status_effects

It might be useful for plugins to know these colors.
For example a plugin might want to spawn a particle effect with the color of a certain potion effect, or color leather armor in the same color as some active potion effect, etc.

A good place for adding them to the API might be as Color attribute of PotionEffectType.