It's just one more example of George forgetting his own lore along the way. Even within the making of ROTJ. Luke's costume was designed to be similar in simplicity to his costume in ANH, but -- in George's words during Mark's test-fitting -- the black was "more Jedi-like".
So when Sebastian Shaw got his Ghost Anakin bit filmed, I scratch my head. He and Alec were substantially differently-sized people, so they didn't just throw Obi-Wan's costume on him. The outer robe, maybe, but even then, that got a dye dip to darken it a bit. The inner robe and sash were newly made, with none of the dirt or weathering Obi-Wan's had. And the shirt had details Obi-Wan's didn't. So that was a deliberate choice made during production and after Luke's costume had been designed. No satisfactory explanation.
Then, later, in pre-production for Phantom Menace, Iain McCaig 's concept art for Obi-Wan (back when it was just going to be him, and Qui-Gon hadn't been created yet) showed him similarly in black and dark gray. But George nixed it in favor of the Tatooine homesteader garb we're familiar with from ANH when Obi-Wan was in hiding.
I stick with the darker palette in my Jedi costume work, though. It makes more sense, in context. Black foundation layers, dark grey tussah sleeveless kimono and sash, dark brown cloak. The whole thing with nominal color schemes for monastic or knightly orders is a certain semi-uniformity to aid visual identification. Trappists can't be mistaken for Franciscans can't be confused for Benedictine is clearly different from Buddhist, etc. If Luke was dressing a particular way to add credence to his claim to be a Jedi Knight to Jabba, that has more weight for me than the Old Ben outfit.
