I've got a problem when printing from LR6 (but also in previous versions and on different computers). Before getting a new computer and a new printer, I've suspected that it is a printer problem but after getting a new computer and a new printer (first one was a Brother inkjet printer, current one is a Canon inkjet one), the problem persists. If I print several pictures, the colors are correct at the beginning, but at some point on a picture, colors become darker.
There is enough ink available and the problem is not related to a specific color. It starts at a straight line after that the colors change. The following schema illustrates the problem (the pictures in the print job are separated by blanks):
CCC CCC CCD DDD DDD CCC CCC CCD DDD DDD
At the beginning, zero or several pictures are printed with correct colors (C). Somewhere in the print job, the colors become to dark (D). The subsequent pictures are printed with darker colors. If I start a new print job, colors are correct again.