Although there are wide variations in flower color at all seasons, I tend to think as summer as being the time of many yellow flowers of the aster family. Hence the derivation of the term for the many often unidentified yellow composites/asters as “DYC,” or darn yellow composites! Here in SW VA in fall we continue to have many yellow flowers such as goldenrod, but it is also the time for spectacular blue flowers, many of which are specialized for pollination by bees, especially bumblebees. Bumblebees are physiologically adapted for foraging in cooler fall temperatures due to their ability to raise their body temperature by muscular contractions. Thus when I go out on an early fall morning, it is cool and there are primarily bumblebees on flowers.
A very popular flower in the aster family for many insects in our September yard is the blue mistflower or ageratum. It has a flat flower shape that is attractive to a wide variety of insects. For example here are some photos of a monarch, a bumblebee and a yellow collared scape/tiger moth on mistflower.
In contrast some of the lobelias in the bluebell family attract specific pollinators. The spectacular great blue lobelia which blooms in late summer and early fall in our area is primarily pollinated by bumblebees. Yet a close relative, the bright red cardinal flower, primarily blooms earlier and is usually pollinated by hummingbirds.
The color blue seems to be an attractant for bumblebees which may not perceive red as a distinct color. Thus the fall-blooming blue curls (mint family) is pollinated by native bees. The fall blooming deep blue monkshood (buttercup family) is a bumblebee specialist.
The more you learn about flowers, the more complex their structures and functions are revealed to be. Yet there are common evolutionary themes that begin to reveal themselves when you look for patterns. One of the more interesting is the relation between flower color, structure and the pollinator(s) for which the flowers are most attractive. So when you go out to commune with nature don’t just smell the flowers, look carefully at them and the pollinators that they attract.