ExtremeMHz has wrote up a guide that explains you howto replace the LED in your optical mouse with another one :
You ask yourself...Why do all optical mice have red LED's? Can I change them? How will it affect it's performance? Well, we will answer these questions for you and provide you with a guide on what is required if you want to change them. Maybe you want your mouse to match with your case lighting? This is a somewhat risky mod but if done properly, it'll have you saying..."Red is Dead!"