Well the portraits don't always have to match the sprites nor do they, for instance, chrono and lucca appear differently in their portraits than they do onscreen (I've done a little work on my own to try and fix the portraits, but I'm having trouble adding patches together... my patch doesn't seem to come out unscathed when adding the re-retranslation by Doctor L...I think my patch was compatible with the first re translation though)
So about magus' portrait, I think it should match the stock magus in a way, as in, matching the level of contrast and brightness. I think if the prophet prt. and the stock magus prt. look too different it won't be as awesome I guess.
Just a random thought.. If you take out some green from the lining on the hood, then you'll get more of the red look that we see on the lower part of the stock magus portrait. Once again, it would be ideal if it matched the stock prt. with minimal changes