There are two very inexpensive ways you can bring plastic back to its former glory.
First of all if you have a heatgun, gently apply heat to the plastic and you'll slowly see the colour return. Be careful not to put too much heat in one place or you'll end up with very unique bumpers.

Secondly... Peanut Butter! Believe it or not, the oils in it are absolutely brilliant for restoring external plastics (although avoid crunchy where possible).
HTH