 You may have noticed certain chips are available in a much brighter package material. Not vintage white ceramic, but a white plastic body. Most often worn by optocoupler ICs like the 6N138 and 4N25, with no readily available explanation. And I'm guessing manufacturers wouldn't suddenly start prioritizing color coordination, outside of PC motherboards at least. But after some digging, I did find a datasheet from Fairchild comparing test results of the same opto isolator in light and dark variants. The white package showed a much lower capacitance between input and output pins, which seems to contribute to overall faster switching speeds. It does make sense when you imagine the dark material absorbing some of the light from the optocoupler signal, and brighter materials reflecting that light.