He was a bit sweaty too, though. If he dried off & the dirt wasn't ground in, it'd have been easy to brush off. Now if you'll excuse me, I'll stop talking, the idea of a sweaty Harrison Ford isn't something I want to discuss.
As for Jedi costumes, I don't read too much into their colour or shade. Look at the preliminary sketches for TPM, or at Anakin's costume in Ep2.