Ground effects? Is it an option for these to be painted? Some mini's seem to have this area painted but still have the plastic fender flares, while others like mine have it going all the way around the car. Is this paintable or would I have to buy a body kit if I wanted that area painted? I don't necessarily mind it but I have heard that they fade and oxidize.

 

The black plastic piece at the bottom of the bumper is commonly referred to as spoiler, or spoiler lip. The other thing is the rocker panel cover.

Both are usually left black, as their textured surfaces do a good job of covering multiple nicks and abrasions from road debris.

Everything is paintable, but the textured surfaces may not look all that good in glossy paint. Smoothing them over would be almost too much work, thus the aftermarket body kits. As long as the aftermarket parts have a layer of clearcoat (some do, some don't), they should last. Subject to nicking and chipping from road debris.

 

The black trim is the rocker panels under the doors, wheel arches, and then bumper lip or trim.

It is very easy to paint and be smooth we've done it many times. Don't use cleaners on it as they can prevent the paint from sticking and come out later.
When we paint them we would simply scuff it with a scotch pad, then a good primer and paint they come out smooth all the time. Talk to your local body guy.