Maybe he's got a dietary imbalance? But if it is, I don't have any suggestions as to what it might be...

Not much help, rainbow!
But I hope it's just a temporary thing and is fur gets back to normal quickly...
Just a thought--we used to see rust color on our lab and on our blue setters when they licked excessively--it faded and discolored the hair and actually made it look coarse. Is Logan licking a lot lately?