Personally, if you're thinking of selling, keep it the original color. If you're thinking of keeping, by all means change the color if you wish.. BUT, be prepared for the numerous comments you're going to get about it not being the "proper" original color.. If you can live with that, then cool. With the fact you have a C500 the way YOU want it, it'll outweight any negative criticism you may get.. And this is coming from a C500 owner..

We only live once, so ENJOY your car !!!!!