Season 1 sees Will Graham mostly in lumberjack plaid and messy hair. He's meant to be presented as a man who prefers simplicity above all else despite his dark and complicated line of work. In Season 3 after supposedly being brought over into Hannibal's darkness, his dress begins to reflect that of his nemesis. More button downs and blazers and shorter haircut. Male characters don't often get the same wardrobe treatment as female characters, but even their clothing must make logical sense for where there character is at in his arch.