A bit of Googling found a page which recommends white alyssum, blue ageratum and red wax begonias. It's one of those pages where the main Google link takes you to a password-protected site, but you can bypass the password through accessing the cached version, here:
http://tinyurl.com/mz8z6t
If you get good at creating the Union Flag you could try it on a larger scale ;-)
http://www.flickr.com/photos/drrcw/3723427150/
Or simply go massive! ;-)
http://www.culture24.org.uk/science+%2526+natu re/environment/plants+and+trees/art50643
Chris