I got sick of checking my phone every time I received a new email. So I made an application to change the color of your phone's LED depending on what email has arrived. It's called Labelert.
It works with Gmails labels. You setup Labelert to correspond 5 colors to 5 different labels. (These labels can be anything you create or currently use) Once you have a label associated with a color, anytime Labelert sees a new email with that label applied it will flash the corresponding LED color.
You can also setup different alert sounds and vibration setting for each color/label.
Also if you are just interested in changing the default Gmail LED color Labelert does this too!
Check it out at Labelert.com
Or use the barcode scanner on your phone: