The colors bother me too. There are too many primary "digital paintbox" colors. It's a Crayola version of the ancient world. I don't know enough about the history of dyes to be certain, but it seems to me that the range of colors for clothing, shield paint, etc. would have been at least somewhat limited, and probably further away from the pure primaries we take for granted today.

BI (and RTW too) has the look of the early Technicolor movie epics, where all the colors were bright, bright, bright. More modern historical movies tend to use a dulled-down palette, which (historical or not) I think looks better. There is also more (simulated) dirt and dust on everything in most modern historical movies. That's another thing lacking in the look of RTW/BI. All the soldiers are just way too clean... like they put on freshly laundered uniforms for drill inspection in a training barracks, instead of wearing the same thing after a months-long march through enemy territory. Dulling down the palette would help kill that "freshly laundered" look.