As a followup to my just posted message, please take a look at this book http://books.google.com/books?id=O0oPAAAAYAAJ&pg=PA134&lpg=PA134&dq=confederate+prisoner+clothing&source=web&ots=Mimpz5RtRQ&sig=9YT_Ah2e_TcZ8HwtM5rzeCASRoc#PPA134,M1 which seems to state that Camp Douglas prisoners received "gray" Illinois militia uniforms. This authors narrative; however, describes a virtual paradise for Confederate in Union prisons, so I'm a little hesitant to accept his other testimony.
Jim