Mascara is usually sold in black, but most stores also sell clear mascara, which gives eyelashes a shinier look than no mascara. It would be cool to have more options for natural mascara though, because the clear mascara is not very visible. (Hence the clear.) Personally, I prefer black mascara, but a variety of colors is always good :)
The standard color for mascara is black but I think it would be suitable for it to be sold in all natural shades like auburn, blonde and brown. I think we should have that option the same way we do for choosing a hairdye that we like. I think a lot of people would like it.
There are alternatives to black mascara. Brown for example can be more warming and is a nice alternative for a lot of people. Purple is another good go-to alternative. There are shades that match just about everyone's eye color (green, blue, brown). You don't always see them in general stores, so they're kind of a specialty product. But the key is mascara aims to enhance the eye and the color is usually chosen based on the color of the eye (either the black of the pupil or the color of the iris. I'm not saying hair color isn't an option, it's just not going to have mass-market appeal.