Might not make as much sense as it seems. Mostly because the canon seems to suggest that once Revan was taken from his flagship after Malak attacked it, he never wore any version of the armor/robes again(not considering that panel from the comics). Even the LS ending of the game refuses to let you wear even the stripped version of the armor, unlike the DS ending where anything you wear during the fight with Malak is shown in the cutscene. Making the cloth parts of the armor brown won't matter much, as most clearly define that armor/robes as his "Sith Apparel".
And they would only show him in those brown robes for any of the TOR material, because they don't want to define his face, and to re-enforce the canon that he goes light-side during the game, and after. It also makes another connection to the movies, just like much of KOTOR does.