All of us have this key called "Scroll Lock" sitting in our keyboards doing NOTHING most of the time except for ( the most important part ) turning an on-board LED on/off. I use my computer in the dark many times and rely on my monitor to illuminate the keyboard, which is not quite good.
So , I hacked my keyboard to replace the tiny dim green indicator led with something else.
Open the keyboard to get access to the on-board LEDs.

Figure out which one is that of the scroll lock and de-solder it (or just cut the leads) .
Solder two wires in place of the leds.

Drill holes on the keyboard and insert the leds.
Connect 2 LEDs in parallel to the newly soldered wires. I also had to trim off a bit of excess plastic on the casing to make way for the wires.

The 2.5V output on my keyboard only allowed me to to connect 2 red or green leds. Other colours wouldn't light up with that supply.
Make sure the connections are OK and put the two parts back together and connect it to the computer.
If everything goes fine, turning the scroll lock on should illuminate your keyboard.

You could also use the caps lock and num lock indicators. But don't blame me if u find it difficult to use your keyboard when u want your keyboard illumination on...

I am a beginner to arduino and electronics.
This is a prototype of a colour changing wrist band made by me . The RGB LED's Color and brightness can be controlled using any android device which supports USB (through OTG).
I used the following (android) app  and source code (for the arduino) .
Arduino Control RGB LED (Google Play)
Arduino Source code
The wiring diagram can be found with the source code.
The android device can be unplugged once the required color is set.
But to use the blink and fade feature of the app the device needs to be always connected to the arduino. 

I used an arduino uno which was quite big for this project. Even under the shirt, the on-board leds can be noticed in the dark. A smaller version of arduino (nano or micro) would have been ideal for this.

Its Working...!

Further i am planning to add an accelerometer to the setup to set the colour using hand movements.