An early theory suggested a tendency for butterflies to use flowers colored like their own wings, relating those colors with mate recognition and protective camouflage. Bees have the very broad range of color vision, but petals or guide marks that are violet, blue, ultraviolet, or yellow will especially attract them. They are blind to red, however. Unlike most insects, birds are quite sensitive to red. A number of plants offer red winter fruits to birds, which will very effectively distribute the seeds along with a dose of fertilizer.
Red flowers attract nectar-sucking birds, as everyone with a hummingbird feeder knows.
The size of the color display appears to have an effect also. Dogwoods have tough, white bracts, which most people think of as petals, functioning the same way. Shape Honeybees, at least, have been studied enough to know that they prefer certain flower shapes. A British study in 1970 confirmed a decisive attraction for star-shaped or lobed forms over circles. Generally speaking, insects are more often excluded by a shape than attracted to it.