redearslider.com

ok i have a slate rock that's a burgundy color and they slide off it into the water all the time(for fun). do do it constantly until they get tired .(i'll get a video) can all that sliding make there belly like dark red?

Do mean if the color is rubbing off onto the shell? If the slate is 100% natural, it shouldn't leach any color.

If their plastrons are having contact with the slate as they push themselves off, I'd watch for signs that their plastrons aren't getting any wear on them. Even stones/rocks that have seemingly smooth surfaces can abraid the shell fairly quickly if it's rubbed over them.

My turtle hurt his plastron by rubbing it around on and sliding off of a piece of smooth flagstone. The injuries were small red dots. They looked like bruises. Could the red on your turtle's plaston be bruising?

maybe... they seem healthy and active and they have no signs of pain

If you think the slate could be causing red areas on the plastron, it would be good to change to a basking area make out of something else. Corkbark is good.