ActionScript 3.0 :: Determine Darkness/lightness Of Hex Values?

have about 100 small square sprites in a grid, they all have their own color that is stored in an array of hex values.I'm trying to assign different z values to each square depending on it's hex value, so the darker the hex value the further back (higher z value) the square is positioned, and the lighter it is the further forward. I guess I need a function that returns a Number depending on the hex passed in, so for example I pass in "ff534751" and it will return a Number somewhere between say 100 for black and -100 for white? I hope that makes sense?