Hi Elsa. I mask the bevels with removable tape and burnish it a little with a boning tool on the edge next to the bevel. I paint the bevel with a small bristle brush and acrylic craft paint. You can paint them any color.
You can put the tape on first and then cut the mat, but I'm better at doing it in the order I mentioned. It's one instance where I'm glad to be so near-sighted. I take off my glasses and can see really well.
If paint bleeds a little, I wait til it dries and then sand it with an emery board or scratch it off carefully with an Exacto knife. (We use Excel blades. They are sharper.)
Sometimes when the removable tape is peeled off, there is some residue that will leave sticky stuff on the glass. I carefully go over that area on the mat with a rubber cement eraser before putting the package together.
And voila, a painted bevel!
Painting bevels doesn't take much time once you get the hang of it. Once again, it's something I learned from The Grumble!