To my knowledge, PRR diesels to be used in passenger service (in later years) were all painted Tuscan red. Your problem seems to stem from the fact that the model manufacturer's version of Tuscan red doesn't match the paint vendor's version. So, you can either re-paint and re-letter the entire engine to match the paint vendor's version or you can buy a version that approximates what the model manufacturer used.
A long time ago, the PRR Technical & Historical Society put out some "drift cards" (basically, paint color samples), but I don't remember if Tuscan was included. I do remember that the issuance caused quite a stir because some of the old-timers thought the colors on the cards were off.
Beauty, and correct paint tint, are always in the eye of the beholder!