Changing my keyboard backlight to rainbow colour

As sig says, that's not directly possible. Some bright engineer sorts may have figured out a way to modify the hardware, for all i know, replacing the white LEDs with colored ones, but that would not be the sort of thing anyone who isn't an experienced electronics engineer should try... Unless, of course, you're okay with experimenting on an expensive device that may end up ruined in the process.

I don't think possible. But keep dreaming.

Actually, I stand corrected. There's an easier way than replacing the LEDs: colored plastic films. This is still not for the faint of heart, and will almost certainly void your warranty, and may ruin your MBP if you screw something up... But if you're still interested, see:

http://www.sealiesoftware.com/keyboard/

The white LEDs can be changed to another color. Shortly after the LED-backlit keyboard appeared, people would proudly show me their handiwork. Generally, the users were photographers, and used a deep red LED to prevent film exposure problems.