Apparently after changing one of the toners in one of the HP 4550s today, I sprayed the cleaner on the printer and it got sucked in. This caused splotching on the color printouts going forward.
After swapping several parts with the "standby printer", I read the troubleshooting guide which suggested the transfer roller. When I got home tonight I swapped transfer rollers and that solved the problem / moved it to the other printer. Fortunately the transfer roller is about $27 from a number of sources and I found a new one on Ebay for $10 (shipping included). Actually the transfer roller probably was on its last legs anyway as I bought the printer used and it now has about 45000 pages on it. The transfer kit including the roller is supposed to be done every 25000 pages or so.