As we don't show Merlin we haven't worried about the tear staining as it's really a cosmetic issue rather than one that is causing any problems for him (vet said that if he is under GA for anything else they'd look at whether it's a blocked tear duct or congenital issue in his nasal canal, failed the tear test on right eye).
All we do is wipe his eyes daily. I had most luck when I used some diluted hibiscrub on it daily (5:1) and it really reduced it within a couple of weeks but still visible.
I know there are all sorts of suggestions from ensuring there is no beet pulp in food to things like angel eyes, have you searched previous posts? It's actually how I first found out about COL because I was searching for info about it!