On most coins, attractive blues and greens are almost always the rarest. A bright electric blue is extremely rare and popular, and a strong neon green or lime green is spectacular. Here are some coins I used to own with the blues and greens I'm talking about:
The toughest tone is probably on a recent coin; many old coins have toning but it’s tough to find a 10 year old coin with any level of color at all. I found that placing westward journey nickels into a Specific Whitman folder Led to some reasonably golden tones