Replacing the LED in an optical mouse is pretty easy if you're not afraid of a soldering iron. My Logitech Wheel Mouse Optical was screaming to have a blue LED instead of the red one. I used the instructions here: and Radio Shack part number 276-316 which is a 3.7v 20mA 2600mcd 5mm clear lens 30 degree viewing angle LED. Instructions i followed were here: although the mouse they modded there was a microsoft white (i.e. non-transparent) mouse such that you would have to look on the underside to even notice that it was modded. oh well. my instructions follow the pictures:

inside of the mouse, low intensity
inside, bright (moving a little)

Basically you just open the mouse and take the wheel out, then take the PCB (short for printed circuit board, I think) out. take the retaining clip off the clear plastic assembly attached to the LED.

Bend the plastic housing up (Carefully, the LED leads can twist in their holes and make the circuit traces get messed up, learned that one the hard way) and pull up on the part behind the LED (using pliers or tweezers) while pulling the plastic holder off the LED (I don't have any very good pictures for the insides but you'll see what i mean if you have the same mouse) carefully unsolder the old LED, noting the orientation of the slit inside (the angled slit is the side of positive, the new LED should go in the same way, no harm if you put it in backwards but it won't work, (pesky diodes))

Put the new LED in with enough room on the leads so it'll bend back into the plastic retainer and have enough room to fit properly, I would assume if it isn't close enough to the lens inside the mouse, it won't track as well. Solder it in, making sure to get good contact but not bridge any places you're not supposed to, and don't melt too much plastic in the process.

put everything back together the opposite of how it came apart (they're not really that complicated inside so it shouldn't be too hard) and plug it back in. initial success will be it lighting back up, real success will be it tracking across whatever surface you use. screw the case back on and go forth with a different color mouse!

at first the blue didn't seem as bright as the red was but i'd assume it's just cause it's a different color, and because it works perfectly fine, i'm not complaining. one post online said someone used an IR LED so they had a dark optical mouse, i think what would be cooler would be a UV LED and a black light responsive mouse pad :)