Since there are thousands of types of mites, you really can't tell just by their color, but...
...here are a few things for you to think about.--and they are the opposite of what you said you heard!
Snake mites are parasitic, so they suck the snakes blood. This makes them very dark to black in color (especially when their bodies are full of blood). I have been plagued by mites 3 times in the past 10 years or so. Every time this occured, it was about 2 months after purchasing snakes at a show--and everytime the mites appeared black in color.
I have seen white insects that are either mites, or just look like them, in grain, wood chips, the lawn, etc. and every time they were very pale in color--and not a parasitic form of mite.
I raise crickets. If I feed them light colored food, they are very pale. If I feed them dark colored food, they are very dark. Anything feeding on the blood of your snakes sould be very dark in color, since the snakes blood is very dark in color (especially when concentrated). Mosquitos, for instance, remove water from the blood they suck up, so they can suck up more of the nutrients they are after in one feeding.--I don't know if mites do this also, but it wouldn't suprise me.
Rodney