I had the same feeling when I started using MSFS 
, but with experience I now know that I was wrong.
There are also white Marshallers, and if I remember correctly, you can see some at EKRN (Ronne) airport.
But it seems to me that it is always “binary”: either they are all white, or they are all black. I don’t know if the character selection is done like for airport vehicles (which depend on the regions), but it lacks mixity, that’s for sure.