Pumpkinseed(PS) and Bluegill(BG) are both species of "sunfish". Sunfish is not a specific species but rather a group of closely related species. There are in fact over a dozen species of sunfish and even the black basses, crappies among others are considered to be in the sunfish family.
IDing species of sunfish comes down to morphology of the fish and less with coloration. As you get better, you can use coloration, but if you use coloration alone, you will make many misidentifications. The best web site for IDing sunfish with is: http://bio.slu.edu/mayden/sunfish/sun_key/1.html
Another good one is: http://www.tnfish.org/FishIdentificationID_TWRA/TWRA_Sunfish_Identification_Key.htm
Coloration in sunfish varies with many factors. Regional variations, water chemistry the fish is found in, the sex of the fish and the time of year it's caught all factor in. Some sunfish species even have subspecies. BG for example have 3 scientifically recognized subspecies. Each has a different coloration, hence the nickname of Coppernose BG for the subspecies of BG which was originally found in FL.