Resisting the urge to post how Voyerism is the cause of PC's with acrylic windows and naked cores...
The energy from the water to the core is not photonic, nor is the transfer of energy from the core to the air. Color won't make any difference. Seriously, if it did, their wouldn't be any black cooling fans.
The fins are not painted, thus painting won't have any real effect on it's cooling ability. The sides and tanks are not in the duct, so any heat released by them would be passive.
Paint would hold heat in, acting as an insolator. Even if the paint were highly thermal conductive, it would still be yet another layer for energy to go thru. However...
Cores are OVERKILL. A single Chevette core with decent air flow would probably cool your computer and three of your friends computers even if it's fins were lightly painted. Painting the fins would decrease it's efficiency, but consider this: A heater core is capable of transfering all the heat needed to keep the inside of a car toasty even while inches away it's snowing. A big enough core could almost be used passively just with natural wind currents thru the room.
A Koolance can keep a dual Xeon rig alive. It *might* even keep a TEC based computer alive. Copper cores have 2-3X the thermal conductivity and even small ones have 5x the surface area. In other words, you could probably put seeds in 3/4 of your Core and have a wicked looking Chia-Pet, without having too much effect on your cooling.
A little paint won't hurt, and well, naked cores are kind of ugly. I think the reason they place them below the dash is so even the mechanics won't have to look at them. A nicely painted core can be quite attractive, especially by comparison.
Paying $8 isn't such a bad deal, especially if you don't want the hassel of adding barbs or talking to AutoZone people (like I actually own a Ford Truck!! Just go in the back and bring me my core, darn it! How am I supposed to know if it came with A/C or not, it's going to map to the same part number anyway! Just go get it!!!)....
