I think it has something to do with the Empire becoming much stricter and trying to eliminate any individuality. At the end of ROTS the Star Destroyers also are plain and have no clor like they did at the begining of the movie.
But, I think it is posible that there
are colored Stormies in the OT. We just don't really see them. If you think back to ROTS, the only white clones we saw were on the Star Destroyers and the dead ones in the Temple. In the OT the only Stormies we see are from Star Destroyers or the Death Stars. The Troops on Endor are from the Death star so that would explain why they are white. The Sandtroopers are a possible "color" variation even though they are just a dirty white. The Troopers on Cloud City were also from a Star Destroyer, so they would be the white "Fleet" Stormtroopers. In ROTS the Troopers that are stationed to a particular planet have different colors, but the "Fleet" troopers are white. This is all purely hypothetical, but it kinda makes sense.
CHEWIE, I like the 501st colens and Stormies, but like DD I don't really like the speckling. It looks odd. I think you did a good job and the chipping on the blue paint for the stormies. It looks more random. I think you could add the blue on the knees for the clones and the stormies. Your lines do look nice and straight, you sell yourself short on that ability.