Here's a little HTML5 game I made in my spare time. It is mobile-web-app-capable, which means if you are using Mobile Safari or Chrome, you can add it to your home screen.

Some people have said it’s kind of a hit or miss, but you can actually improve your guess by estimating the intensity of each RGB channel. I plan to write a detailed ‘How to Play’ explaining the basics of hex and other numeral systems.

You can play the game here and view the source here. My personal best is level 15.