A friend of mine, Don Gabelhouse, who is Nebraska's fisheries chief has told me that muddy conditions during the spawn do, indeed promote hybridization of lepomis species.
BG/RES hybrids do occur sometimes in the wild. I think I actually have one in one of my small ponds. My guess is that they grow pretty big if they have appropriate forage.
Both male and female bluegill have more vivid coloration in clear water. Sometimes after a big runoff event, and the subsequent clouding of the water, our bluegill will turn almost silver in color due to turbidity.
Also, stragely enough, bluegill that are transported in dark colored containers are brighter on arrival than those transported in white containers.
