First lay out some newspaper/tissue on a surface you don't mind getting dirty. (I've used an old princess clipboard :P)
Choose the key you wish to colour, and take it off your keyring (or lanyard) so the key is completely separate
Lay the key like this⬆ on the surface
Wipe awash any dirt (that pink stuff in the crease of the key is the reflection of my pink iPad, don't mind that)
Choose a nail polish you wish to colour the key with. I chose this pink sparkly nail polish.
Begin to paint the key with the nail polish. You can paint as many coats as you want
Tada :) now wait patiently for it to dry so you can do the other side. It may take up to 20 minutes to dry depending on how thick you did the coat
Now do the same with the other side!
Before and after!
The creator of this guide has not included tools
~In my belly button~