Yes, I have seen brass start to show through on sliders before. It is ok as long as they are not in real bad shape. My current set has this occuring.
I recommend you have your rotors turned to minimum thickness, clean sliders and area the sliders set in, and assure the calliper itself is not sticking.
We spend a lot of time after every race on our brakes and it seems to pay off. Brakes don't drag much and they stop on a dime.
If one side is binding and the other is not tear them down and compare, get familiar. You will figure it out!