Haha you can use the flag of the parent nation if you want. Or you could use the parent nation flag and add something small just to highlight the colonial status. E.g.

[img=http://www.crwflags.com/fotw/images/z/za-cape1.gif]
Cape of Good Hope (British Colony)

[img=http://upload.wikimedia.org/wikipedia/commons/1/16/Flag_of_Colonial_Annam.svg]
Annam (French Colony)