Plant colour
Plant colors are very noticeable, especially in flowering plants, and especially the color green. Plants are green because it is a functional color. They photosynthesise, and the chlorophyll they use happens to be green. However, flowering plants show many other colors. Blue is quite rare, though bluebells are a famous exception. Apparently the color is difficult to make by the usual plant biochemistry. It is also rare in animals, so if it is difficult for plants to make, presumably it is difficult for animals to make.