Union regiments generally had two flags the Regimental Flag and the National Flag. Both are interesting. The Regimental Flag, in the standard pattern was a gold eagle holding a scroll banner with the name of the regiment, although there were exceptions. The green flags of some of Irish regiments come to mind. The National Flags were just that the Stars and Stripes. They are interesting because they were individualized by having the name of the regiment sewn on as well as the names of the major engagements the regiment fought. Many flags suffered battle damage, which I don't beleive was ever repared, damage being a mark of honor.
At the end of the war the flags were returned to their home states, where they usually suffered one of two forms of benign neglect. Some were put on public display where they slowly crumbled to dust. Others were cased, rolled on to their poles and coverd with a canvas casing, and stored where they slowley crumbled to dust. Many have been completly lost. Only in the last few years have serious restoration attempts been made.
So far as I know no comprehensive compliation of Colors has ever been made. Your best bet is the state historical society or museaum. Keep your fingers crossed. I was trying to find pictures of the flags of the 35th Ill. Vol.
Inf. and although I can get descriptions, the flags them selves are less than 35% remaining and there are no pictures.