Yes, the name Maroon Langur really is a poor descriptor, I've become a bit desensitised to it at this point though despite the fact that the Maroon Woodpecker occurs in the same forest and that genuinely is maroon.
The alternative name for this species, the Red Langur, is probably far more appropriate. It's about as red as a Red Fox or Red Squirrel, but Maroon Langur seems to be the name preferred in general.