It really depends on what kind of mat it is...
A plain black rubber mat can look more welcoming if it's painted, and painting rubber mats might not be as difficult as you think. Depending on the kind of wear it gets, you can use primer and exterior house paint or specialty paint from folks who really know rubber--tire and auto accessory dealers.
You can paint it no matter what the material. Here are a couple examples from other Hometalk members. Wishing you the best!
There are more here. Just scroll down to find them.
I would use an outdoor spray paint . I would probably paint it a solid color and then make or buy a stencil and spray over it using a second color .
Chalk paint works on door mat but you will need to seal it https://simplytodaylife.com/custom-doormat-with-chalk-paint/