did your original brass radiator come painted?

I would say it depends on what kind of paint you use. regular spray paint, you should be ok.

if you use a ceremic based, engine paint, I could DEFINITELY see that affecting the heat transfer rate of your radiator.

I painted the aluminum radiator in my Dakota. flat black, then laid a stencil on it, and sprayed the Mopar "M" in blue paint over top of that.

looks good when the hood is up, and I don't notice any problems keeping the engine cool.