Use 2–3 Basic Colours: that is limit of the number of colors on the flag to three, which contrast well and come from the standard color set.
Not altogether sure I personally agree with this one. While some colour schemes can be done to make the flag look a mess, especially some local and county flags - whereas one of my favourite flags, the flag of The Seychelles, manages to incorporate five colours and still make it look classy.
I agree that this is probably the shakiest of the canonical flag design rules. The flags of Namibia and Comoros look great with their five colours, while the flag of South Africa (my vote for the world’s best flag) and the rainbow flag look even better with six.