It looks like by "colorful signals" they mean colorful surfaces of animals and plants, in the case when those colors are not colors necessary for the function o

It looks like by "colorful signals" they mean colorful surfaces of animals and plants, in the case when those colors are not colors necessary for the function of some vital organ, e.g. the pigments that capture light in plants or the red iron-based or blue copper-based respiratory pigments (such pigments have appeared before vision organs).