Some of the German Navy ammo is quite colorfull but rare in the US. German artillery ammo through WW1 was nicely painted as well. They dropped most of this in favor of lettering stenciled. This was due in large part to their soldiers being able to read. In WW1 and prior many soldiers from most countries could not read and so the color codes were needed.
The private school which my children attended made a great deal of money teaching US soldiers how to read. I am sure that many others did as well.
To my knowledge the British and the Italians had the most colorfull artillery ammo of WW2.
Today, some of the French ammo is extremely colorfull.
Here is a small look at some of the British color from one of our publications.