My experience is that the most vivid colors come out on 'Michael's Golden' when in full sun. That being said, this one suffers from full sun in the drought season here in the Pacific Northwest.
Here it is in full sun in October of 2014. I would take a new photo today, but I recently moved them to a bed with some morning shade and they aren't to happy looking with all this rain.
I do feel that full sun brings on more intense colors in almost all of the semps. Then there are semps that seem to show more extreme coloring when stressed by heavy spring rains. Like this heuff.
Spring and early summer does seem to bring on the color show in many of the semps.
May
June This one is blessed with great color year round.
June and October
There are so many variable between the hybrids. Some have the best color in spring, some in summer, and then there are those with great color changes throughout the seasons.
But I do believe that day length, temperatures and exposure to sun affect how well each cultivar will color up.
