This question came from Quora. My answer:
The UK flag looks pretty good in black and white. The transform shown here was done in Photoshop, keeping the values (the relative lightness/darkness of the colors) consistent.
This flag predates not just color photography but photography itself. And other flags dating to the same era (18th/19th century) are also very contrast-y, so that they could be easily recognized at a distance. Red, white and blue were popular for many countries because they were three lightfast colors that could be made relatively cheaply and which provide a dark (blue), a middle (red) and a light (white) color.