why aren't there Yellow Dragons?

[hamishspence]

I think old Dragon magazines had additional metallic dragons as well. I remember a Crystal Dragon? And Shadow Dragons.

Believe me, if you go back 35 years, you will find just about every dragon imaginable.

Crystal dragons (and the other 4 common Gem Dragons) all made it into 5e, albeit rather later than some of the others (in the Fizbans book).

Shadow dragons received a bit of a makeover for 5e in the MM, becoming a template rather than a dragon type. Annoyingly, it's mutually exclusive with the dracolich template - so you can't have shadow dragon dracoliches, even if there were adventures in old books with them in.