If you've done LED mods on your Xbox, you won't have much more difficulty doing a GC. You only have a 12V source so calculate your resistors accordingly.
Use
this resistance calculator if you're stuck.
If you don't want to add new LEDs and just want to change the red power LED to another color, just remove the red one and replace it with a 5mm or 3mm LED of your choice.
If you're looking for suggestions on LED locations, you can do what I did and install LEDs around the side vents. It adds a nice glow to the inside of the case when you're in a dark room.
Thats what I figured.
As for placement I'm working on a clear case (from Xecuter) and I'm trying to figure out where good placement is. I know im gonna but 4 or 6 under the jewel but im not shure about a underglow.