Is this correct with the use of which?

No. A pronoun needs an antecedent. The reader has to go looking for the antecedent of "which" and picks the wrong one (to serve) at first.

"Upon her rise to power, Beryllinthranox allowed a number of lesser dragons to serve her, both green and red." That's if the reader already knows about dragon colors. If not, "Upon her rise to power, Beryllinthranox allowed a number of lesser dragons t

I like your answer, except this:

enoonboth green dragons and red.

It is probably better if it's: both green dragons and red dragons

HalfMoonI like your answer, except this:enoonboth green dragons and red.It is probably better if it's: both green dragons and red dragons

Why?

It balance out the sentence out better.

"...Beryllinthranox allowed a number of lesser dragons to serve her, both green dragons and red (what?)."

It's the same reason the sentence below is consider unbalance:
"Tom's paycheck is less than Sue's (what?).
"Tom's paycheck is less than Sue's paycheck."

It balances out the sentence out better.

It's the same reason the sentence below is considered unbalanced:

Balance is not the issue. Sing-song repetition is. It is the writer's choice, but I recommended the version with the tac

HalfMoonboth green dragons and red (what?). "Tom's paycheck is less than Sue's (what?).

Those are just fine. No native speaker would be thrown off by those omissions.