This is something a lot of shooters don't seem to give much thought to. The color and configuration of a target can have a considerable effect, not only on where your groups print but on how large they are. This is especially true with open sights, but can also make a difference with scope-sighted guns particularly for those of us with older eyes.
I am currently researching an assignment for 'Handguns' magazine on prescription shooting glasses and will speak to the Optometrist concerning this subject. It would be interesting as a side note in the article or may be worthy of a column in it's own right.