there is few powder coating, I used epoxy power coating for laboratory stuff. But bare in mind, there are few type, normal one esp white will turn into yellow if put under the sun for long time. I think there is some type specially for out door one. You must check with them 1st.
The powder is not too expensive, but the workmanship will be expensive since you only be spraying 4 rims. They need to set up, clean up the sprayer, remove the previous colour, put in your powder....etc. AFAIK, the powder only sell in bulk only.

can't get 100% matching. Even with the same colour code, the colour got slight difference in every batch.

RM200 for 4? That's too cheap to be true. Spray maybe yes....