Water does hold heat and a big lid on top holds that heat in. However, Those temps aren't dangerous to your fish. Crappies and Bluegills survive all over the country in crazy temperature ranges. I don't heat or chill any of my native tanks, I let them just stay around room temperature, they get up to the 80's in the summer and down to the 60's in the winter and my fish do just fine.
As far as coloration goes, there are several factors. Mine color up beautifully when it's time to feed, they get really dark and their patterns are beautiful. I had an LMB that would get a super dark line and very splotchy during feeding time, like he was trying to camoflauge himself from the food, natural instinct. I have found that for a few weeks after putting WC fish into a tank, lights off will stimulate better colors and more activity. I leave the lights on during the day when no one is home to just get them used to the lights. When I get home, for the first few weeks I leave the lights on with me in the room for about an hour or so, then turn them off for the night. Eventually they get used to having me around and become much more active with the lights on. Just be patient, remember the fish are coming from larger bodies of water to smaller tanks with crazy people staring at them all day!