I want the left side to be green and the right side to be gray. As pictured above would be PERFECT. Preferably a pure CSS solution (only need to worry about WebKit).
Is such a thing possible?
Yes, it is possible. Though I wouldn't recommend it because
input range is not really supported properly by all browsers because is an new element added in HTML5 and HTML5 is only a draft (and will be for long) so going as far as to styling it is perhaps not the best choice.
Here you go: http://jsfiddle.net/JnrvG/1/.