Colors, Patterns, Genetics, Etc

There is no general rule I know of regarding color names. But from what I have seen so far people tend to name them as follows:

For regular scales - usually the first color refers to the body and the second to the fins.
Eg. a multi blue red = blue body with red fins

Dragon scales means they have scales that look thicker than regular colors/scales. For dragon scales, the color refers to the fins because most dragons have light colored (whitish or silverish) bodies .... except for irid colors like green and blue - their whole body and fins are green/blue.
Eg. red dragon = white body with red fins, black drgn = white/silver body with black fins ..... and so on.
But for multi colored dragons, the same principle applies: multi-blue dragon = blue dragon body with (often) red fins (both full or partial)

Mustard gas (MG) was originally a blueish body with yellow fins (both full or partial). But nowadays many breeders would call what ever body color with yellow fins as MG.
Eg. green MG, copper MG etc.

The problem is that many breeders give fancy names to their bettas which often is confusing. In such cases you would have to know/understand that breeder's "language". Can't think of any examples at the moment - I've never really bothered with it.