I don't think that's a 100% inaccuracy. Unless there's some new research I don't know about*, bright, colorful clothing was a scarce commodity through much of civilization. Consider -- you have to bleach the fabric; even cotton isn't super-white straight from the field. Then you have to concoct dyes, and master the chemistry of not only making the dye, but of making it at least somewhat permanent. The same problems apply to paint -- heck, PPG and the other paint companies STILL spend millions on research every year.
It's not that color didn't exist -- just that it was expensive and difficult to produce. We know now, for instance, that the Parthenon was painted in bright colors, and that was the general rule for the marble temples and such, but those were very special buildings.
* Not impossible. I studied costuming and set design in college, which would put it --- hmm, let's see --- yes, in the Grant administration.