It's red in ANH. Yellow in Empire. In ROTJ he wears these clothes when he's on Tatooine, and for the rest of the film he pretty much wears the same thing he did in ANH.
In Empire Han has two costumes, the Hoth protective gear and his "Bespin" clothes. Only the one set of pants has the yellow stripe.
I rather doubt pouring tons of hours into changing a yellow stripe to red is high on Adywan's list, but it the interest of putting the idea out there, well... there it is.