I wish we had a Colors and Markings forum, but I guessed this might be the best or most appropriate forum for this post. See the link below for everything you wanted to know about roundels but were afraid to ask. I'm sure there are omissions and errors, but it looks like a pretty good basic reference. Most of the information/photos are from WW2, but there are other eras covered. Many of the color photos appear to have been colorized, and some have been published before, but many were new to me, and I hope to you, as well. Enjoy!
Mike
http://www.vintagewings.ca/VintageNews/Stories/tabid/116/articleType/ArticleView/articleId/382/Roundel-Round-Up.aspx