Green is the most common floral color. It only seems to be rare amongst flowers because almost all green bloom relies on wind for pollination. Thus, neither color nor fragrance is useful to get the attention of pollinators. Actually, green flowers do not get much attention at all. They are easy to ignore in the wild, and generally unpopular within home gardens.
Most showy green flowers such as zinnia, chrysanthemum, hydrangea and gladiolus are progeny of unnatural breeding. Showy but naturally green flowers such as hellebore and orchid are merely incidentally green, as they employ infrared or ultraviolet color to attract pollinators. Although people can not see infrared or ultraviolet color, many pollinators do.
After all, flowers bloom only for pollination. Many customize color as well as fragrance to appeal to preferred pollinators. They are merely incidentally appealing to people as well. People breed flowers to be more appealing to people, even if unappealing to pollinators. Nonetheless, even breeding is limited to characteristics that initially attracted pollinators.
It is impossible to identify the most common color among flowers that rely on pollinators. Pollinators are regional. Therefore, red and orange flowers may be more common where hummingbirds who prefer red or orange are more common. Purple flowers may be more common where bees or certain butterflies who prefer purple are the dominant pollinators.
Yellow seems to be the most common natural color of flowers of North America. Red and orange are very common as well. Although common, pink is merely a tint of red, so is not a real color. Neither is brown, which is a shade of orange. Although very common among flowers that rely on wind pollination, it is quite rare among flowers that rely on pollinators.
Blue is the rarest natural floral color. Many flowers that seem to be quite blue are actually purplish. Purple is uncommon, but not as rare as blue. Ultraviolet and infrared get almost no consideration since they are invisible to people. However, both are common amongst most showy flowers, particularly white and maybe red flowers. Red is invisible to insects, though infrared is not.