Here is a colouring page for the flag of the US Virgin Islands. Just print, and off you go!
US Virgin Islands Flag Colouring Page
This complete set of flags of Europe colouring pages includes a linked index to make it very quick and easy to find and print the one you want. All are in the correct flag propotions too.
Here's a colouring flag of Oman to print for the kids. The central and left section is a bright red, while the crest stays white and the bottom section is green.