I have heard and seen this issue with dark automotive waterproof
sandpaper used on light colour woods. I follow a process I read in a
posting somewhere.
I clean the wood after using the paper. The colour is bits of the dark
grit getting caught in the grain. Some people clean the wood with a
cloth wet with alcohol, others blow the wood clean with compressed air.
I wipe the wood down with a piece of very fine scotchbrite pad, first
while it is turning, then I stop the lathe and wipe in the direction of
the grain. I usually finish sanding with an extremely fine
non-waterproof paper.
Hope this helps,
Brad
HardingPens.com