The dramatic history of the American Civil War contain lots of "celebrity" units such as the Black Hat brigade, the Irish Brigade, the Stonewall brigade, Berdan's sharpshooters and so forth. Looking at the roster of Bull Run, I found the following which might give me a reason to include a bit of color in the ranks.
11th NY Volunteers, "The Fire Zouaves"The Fire Zouaves were composed of firemenm and apparently didn't wear the normal Zouave uniform at Bull Run, but red shirts and black trousers. I think I'll paint some Zouave figures using those colors and I think it'll look just fine.
1st Louisiana Special Battalion, "Wheat's tigers"These soldiers wore a uniform closer to the typical Zouave clothing. There wasn't awful many of them on the field during the battle, only some 5 companies if my reading material is right. At 1:200 scale that isn't many figures, but I think I'll inflate their numbers to three stands for visual effect.
69th NY infantry, "The fighting 69th"The first regiment of the Irish brigade saw action in Bull Run already. As far as I know they wore the usual union uniforms but they have that nice, green flag which I will definitely include.
On the subject of flags, the confederates did not march into battle with the classic confederate battle flag, but with the CSA national flag, which caused some confusion during the battle as the flag was confused with the stars and stripes of the union. Also, apparently Louisiana troops marched into battle with a very colorful flag indeed. I might as well do my confederate figures in a manner so that I can change their flags (I have an idea) so I can include at least some of the historical flags into the game.
There was also some confusion with uniforms as some of the union soldiers fought in grey uniforms and some of the confederates in blue. I think this can be easily modelled by swapping some of the stands between the armies.
If by any chance you know something I don't about the visual look of the armies at Bull Run, do tell. Just remember these are 6mm figures I'm working on :)