For what it's worth...
I have an article on this topic with some great pictures showing the difference between,
- Single stage
- Tinted clear
- Clearcoat that's stained - The point being when you buff it you shouldn't see any COLOR coming off the car and if you do it's probably a brown/grey/blackish color and that's a sign of dirt stained paint.
Single Stage Paint - Tinted Clears - Basecoat/Clearcoat - Embedded Dirt
Here's just ONE picture from the article from the section on tinted clearcoats.
Here you can see the tint in the tinted clear coming off the car as we compound it and onto the face of the pad. This is nothing to worry about, keep in mind when you're doing any correction work on a clearcoat paint system you too are removing paint that is building up on the face of your buffing pad but because the paint is clear you won't see it build-up on the face of your pad.
And for any of you that read this into the future and wonder how paint can get dirt, check out this article, it explains the issue in detail with pictures that tell the story...
Road Film - If you drive your car in the rain your car has road film
The dirt you see on the pad in my hand... (the pad started out white)
Came from this CLEARCOATED car...
The paint is clear and the brownish color is years of dirt staining that builds up
ON the paint and doesn't wash off. It does come off when you use a compound, polish or cleaner/wax.