Haven't had any custom work done but spent 2.5 years in Junior College to learn the auto-body and fender trade so there might be someone here that has a little more experience than me.  Also I read alot of car magazines to find out what others have done.

Most paint jobs today are done in 2 stages meaning there is a base or color coat which is dull and a clear coat which brings out the color and shine and protection.  Sometimes to bring out different effects one could add metallic flake to the base coat to get a sparkly effect. There are all different sizes and colors of metallic flake. One could also add crushed pearl powder to get an opalescent effect.  Think of it as looking at the inside of an abalone shell but more transparent.  Different results will happen depending on how much powder is added to the clear coat and what the color of the base coat is.  Generally a pearlescent paint job is done on a white or pastel color base coat.  Without seeing the vehicle it is hard to tell just exactly what was done.

Did it change colors all at once or did one color fade into another? 

Any experienced auto painter should be able to show you what kind of custom work he can do.  Also watch TV shows like Overhaulin, Pimp my Ride, Orange County Choppers and Monster Garage.  Sometimes they will go into detail as to what they are doing.  Overhaulin, especially.

I would not reccomend doing this to an everyday vehicle as the cost is expensive and if the car is damaged in any way it is almost next to impossible to get the color to match without having to repaint the entire car.

So I guess the term you are looking for is opalescent or pearlescent paint.

Not an expert just a novice when it comes to these things.