I have noticed a lot of black cats are very soft... but I think it's mostly just that they are very well cared for! Also black fur tends to absorb a bit of heat which can make them feel warm to pet, I love that feeling.
My ginger and white cat is extremely soft. He's a long hair so I brush or comb him every day, which makes him even softer.
We have a black dog, and her fur is a little bit wirey to the touch, but also very shiny. She is lovely to pat, anyway. I think softness mostly depends on breed, diet, how well they're taken care of and if they end up with smaller or larger diameter of hair. Ooh, and age. Someone bought an 8 week old puppy into my work for me to play with, so ******** soft... it was amazing. It was like velvet but cuter and better. It had loose puppy skin and funny giant paws too.