It's fairly simple.
The bulbs have a silcone boot over them that gives them their color. You can buy different colors at the dealer. Then just disassemble as if you were going to replace a bulb. If you want white light, just remove the boot that is there. If you want a different color, remove the amber boot, and install the boot you want. If you find that your guage has a newer "micro" bulb, and you don't have the right sze boot, just install the boot loose over the bulb, it will still have the same effect.
Harris