Don't you find the greys always roll the most... we have 5 horses 2 dark bays, 1 red bay, I blue roan and 1 grey and when we uncovered them during the grey - you guessed it the grey is the filthiest...
I have a black and white mare that the minute I am done grooming her (which takes hours!) will find the nearest muddy spot and roll until she is caked again! I think she just likes to wear mud for her outerwear.
Yes! Omg my gray mare is so nasty right now. We have red dirt so she ends up orange. She looks like an orange popcicle. She loves to roll. Even if there is the slightest sprinkle she is on the ground. My bay...not so much.
Funny story. My friend went to look at horse. He was a little dirty when she rode him and the lady apologized and said he's a shiny little chestnut when clean. He was a good boy so she took him on trial. He rolled in a mud pit so it was time for a bath. After 3 hours of scrubbing, turns out the horse was dark grey. He's registered as a chestnut...