Yes, you can delay tooth decay, but once you have a cavity it makes it all the more hard to keep it from growing, You need to take extra good care of your teeth. If you don't, and just brush regularly, then it is more likely that cavity will grow than you will develop a new one.

You speak just like me, I am about your age and suffered from depression. Happiness is a rare commodity, but Omega 3 helps me get there.

Totally understandable. I suggest you consult another dentist. I can really see no reason why they can't match it to the color of your other crowns.
Check out though the different types of crowns: http://the-dental-makeover.com/dif ferent-types-of-tooth-crowns/ because if yours is from bad materials it might be harder.