I'm dealing with this too, on our sliding door sill (though, oddly, not the front door sills). It's on my list to figure out how best to approach (i.e. paint, buy replacement part, create a new alternative part, etc.).
Our sill also has some adhesive or similar spilled on it, which fills in some of the pattern in the step area, so it will likely look funny if I paint it. I'm probably going to try anyway, just to learn some things about the toughness of a few of the plastic paints that I have.
That said, that sill is on my "long list" to remake in some hard, rubber-like material. We'll see.
Meanwhile: does anybody know if any of these sills (side door, front-left door or front-right door) are still available as new parts?