During the American Civil War battle First Bull Run / First Manassas, regiments of both sides turned up in either the gray militia uniform or the blue 1858 regulation uniform, in addition to any zouave or other quaint uniforms. The clear-cut division between Blue and Gray was not present yet.
Is there any comprehensive source for what regiments wore what uniforms in the summer of 1861?
The website of Library of Congress provides us with a following description regarding the overall problem with uniforms during the First Battle of Bull Run:
As Union and Confederate troops met for the first great land battle of the war, confusion was the only constant. Lack of standardized uniforms made it difficult to tell friend from foe.
Still, particular army units had their own uniform standards. Some of the nicknames of those units come from the clothes, f.e. Red-legged Devils, which points to the red trousers of 14th New York State Militia uniforms.
There's a website http://www.firstbullrun.co.uk, which is dedicated to soldiers of this particular battle. You will find there a lot of interesting informations about uniforms worn during the battle by particular military units. It provides excellent choice of quotes from valuable sources, which in many situations talk about the uniforms. Everything is divided according to a division, brigade and particular unit.
Also the website contains paintings of each unit, that were done with full respect to those researches.
Here's an example, for 2nd Maine Infantry (1st Brigade, 1st Division of Northeastern Virginia army):
"After going a short distance, he saw ahead of him in the woods some men wearing a grey uniform (quite a number of our northern regiments at the time wore a grey uniform very similar to that worn by the rebels, the Second Maine among the rest)."
"The uniforms were grey throughout with drab felt hats, regulation pattern."
The website is full of such descriptions for various units, so you'll find what you're looking for depending on what's the exact aim of the research.
Also be sure to check their reference materials, there are some good books recommendations for you.
I've been unable to find a really comprehensive source for the First Bull Run / First Manassas uniforms. Hence I've started to collect information on uniform colours for each regiment, weapons, strength, etc at Forward to Richmond look under the Painting Guides category or browse the order of battle on the right hand.
Ron Field, Rally Round the Flag, vol. 1 and 2 for the Union is excellent.
With a Red Hat, Red Shirt, Tan Coat with red lining and silver buttons with black cuffs as the upper half of the uniform the bottom half consists of a black belt with “Premiere” written across it with Light Blue Pants.
