+1 to what Dave said. The light part itself isn't super hard. The hardest part would be figuring out where to mount the LEDs. The mounting part is harder. The reason to actually send it to somebody is that presumably they wire it so it's drawing power when the amp turns on. They might be adding a switch on the back for on off too. Doing lights with their own power chord is pretty easy. Integrating it with the amp is going to involve a bit of routing or drilling in the chassis and/or headshell.
Anything more advanced probably involves ICs in addition to lights. Those are pretty cheap too.
A lot of the time you see those mods, they are just using PC case fans. Which is super simple. Mounting two PC Case fans to the top of the headshell, or one on either side is a pretty low intensity mod, and if you ever want to take them out you are just left with holes on the interior of the headshell.
That said, I wouldn't do the light thing. I think it looks cheap and if you ever want to sell it it might limit your audience.
Also, depending on what you go with, you might not need discrete ICs, a lot of the color changing bulbs come with integrated ICs in the bulb/diode housing itself.
Originally Posted by bigdave
The ones that actually change depending on what you are playing? Or the ones with ICs involved that just have shit on a timer? I've definitely seen both, but I've never seen one in an amp that changed with what you are hitting on the guitar on a guitar amp, although obviously it's possible with the right ICs/wiring.