UV stability is an issue with ANY pigmented coating.
clear top coat helps.

Some things to consider are powder compatibilities, and durabilities.
like paint powder is "use specific", meaning you will want to use a powder with good UV stability to start, and again I always suggest a clear top coat (make sure your coater knows they are compatible)
I always suggest finding a local CUSTOM POWDER COATER with a good reputation.
as most production coaters while slightly cheaper, usually don't know the intricacies of custom parts, and/or proper powder for application.

We had a sign on our roof 24/7/365 for 6 years that faded 20% in color difference with NO clear coat.

in 7 years I have only had 1 complaint about UV stability. However the client had the parts on a semi that was outside all the time, and spends alot of time on the west coast. The color difference was about the same 20% lighter, he didn't want the clear coat 5 years ago, and now is going to have the parts refinished as a result.

spend a little more up front to get what you want, and find a qualified coater to do the job