Ask HN: Anybody know how to compare two colors in JS?

A feature in our system allows our customers to edit the background/foreground color of a page. But we need to have checks in place to make sure that the text is still readable.

Given two hex colors (eg: #ccc and #999), what is the best way to compare the colors in JS? Need to return an int 0...n (where 0 is exact match).

I have tried converting to decimal and checking the difference...but this isn't ideal. Couldn't find much on Google either.