Because silver is the bare aluminum, it's cheaper to make them slightly bigger than go through a painting step. Actually anodizing would probably be better since it's thinner. I built this power supply about 40 years ago, it mostly ran hot until I painted the heat sinks black. Nothing else I built 40 years ago is still running.
Anyway it's too bad there isn't a BM1387 stickminer, or something in the < 100 watt range.
I know this is out of context to the original subject..
if you did a finger test to see if the heatsink is colder to say its running cooler, you have done it wrong.
paining a heatsink puts a layer of insulation on the thermal conductive material. you are not "Black bodying" the heatsink. the hotter the heatsink, the better it is conduction the heat away from the source.
you need convection of the heat to the air, not radiate it, its not in vacuum.
now if you google black body radiator, there is plenty of people talking about black painting heatsinks for radiating heat..