Our black and white cat does this too!

Turns herself gray! Up north she rolls in the dirt where we feed the deer! Sandy grey color...

I have often thought they do this to get some minerals from the dirt. It is fruitless to protest...your cat won't listen I bet!
We call it "dirty dirty"...
All 3 of our cats roll on the sidewalk at the edge of our property...and we always thought it was a "marking" behavior...since our oldest will punch out either of the others, if they do it.
But our black and white kitty will pick the only sandy dirt in the street or up North, and deliberately roll in it. Blech!

Here is a funny graphic I found of a cat who looks like ours, doing it in the street:


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