Without a doubt it will void your warranty.
I plan on doing this myself on the G75 within the next couple months when I get more free time.
Without taking the laptop apart and examining it, it is hard to tell right no where the best locations to tap into the power will be.
Likely the safest place will be coming from the battery to the motherboard then solder some wires leading from there to the rear exhaust fans and connect your back light LED's there. There could be potentially a lot of places to tap into the power though, but I think straight off the battery itself is the safest and cleanest.
The keyboard LED color change will likely be straight forward, but a lot more work. Just de-soldering out all the current LED's off the board and soldering in new ones with the color you want (blue, red, green or whatever). You will likely have to examine the type of LED's they are using 1st so you can buy properly matched ones to replace them.
Another Lighting option I am considering beyond these 2 will be some sort of under monitor rim lighting. Perhaps use a LED strip or EL tape, so that just underneath the monitor screen [as it sits open] you can see some ambient glowing from the bottom (I think this lighting tweak would look the coolest from the front on. Without a doubt this will be the trickiest install due to trying to source the power for it. It will clearly have to come from the monitor panel from power already being routed up through there, or require a lead to be routed up around the hinges into the base portion of the screen bezel. The bottom underneath part of the bezel itself will need to be slightly drilled out to accommodate the power cable for the lighting strip too (but it would not be a big deal for that and would not even be visible to the naked eye if done correctly)
EL tape is probably the best for that one (due to slimness), but they require a power inverter to use them and this would not be easy to mount in the free space of the laptop. LED strip would be best to prevent that inventor mounting drama, but I would need to find an extremely slim LED strip to mount there. I still think I want to pursue that tweak just for the cool factor/challenge of it.
Good example of LED Strips
http://www.frozencpu.com/cat/l3/g6/c77/s126/list/p1/Lighting-LED_Lighting-LED_Strip-Page1.html?id=3k...This is EL tape
http://www.seeedstudio.com/depot/el-tape-blue-1m-p-1183.htmlI am not sure yet whether or not I am gonna go blue or red themed with all this yet though...Leaning towards blue currently.
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PC Setup:
Battlestation Setup: