It depends on the type of paint used. There ARE paints that not only won't affect convection, they will increase it, but these are extremely, EXTREMELY expensive. Most standard paints that consumers have readily available contain varying levels of solids in the form of latexes, polymers, glasses, silicates, etc. These are ALL insulators. Practically any paint you buy as a consumer is going to have an insulating effect ESPECIALLY if it already has a layer of paint or powder coating on it to begin with.
No, the change might not be earth shattering in every case, but even a couple of degrees is huge considering people pay hundreds of dollars in aftermarket cooling products trying to DROP a couple of degrees. It just doesn't make much sense to intentionally go the wrong direction when it isn't necessary and when not choosing to simply go with a white product to start with in order to opt for a better performer is perhaps somewhat mitigated by losing those gains due to unnecessarily insulating your radiator. Not to mention, there are really no good products that will look as good you can use for painting the lines and you probably would not want to have to paint the pump housing at all. If you're really worried about aesthetics it just doesn't seem like a good option because no matter how good a job you do it's never going to look as good as simply buying a product that already meets your needs.
In cases where no such product exists, at ALL, when then, we do what we gotta do. LOL.