Here's a little HTML5 game I've 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 said it's kind of a hit or miss, but actually you can approximate your guess by estimating how big is each one of the RGB channels. I plan to write a detailed "How to play" explaining the basics of hex as well as other numeral systems.

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