Blue. I saw the movie in the theater (they had a theater run in the early 70s) as a kid, and even though we had to leave b/c the stepsisters were freaking me out too much (sensitive kid), when I played Cinderella as a kid, it was when I wore a blue dress.
We have the DVD and VHS...we'll pop both in later today and see what the color looks like between the two.
(why my husband owned the VHS version of Cinderella before we met and had a kid I will *never* know...)