Why are olives different colors? What is the red thing in the middle of green olives?

There are different colors of olives and there a different kinds of olives.

The red thing is pimento.

https://en.wikipedia.org/wiki/Olives

Black olives are usually the ripe version of green olives. Pimento is red pepper.

I think "purrrr's" claim deserves more attention:

What is the difference between green and black Olives?
Unripe olives, which are pear shaped and green, will change to a dark purple and then black as they ripen and "fill" with oil. In Spanish this maturation process is called envero. Green table olives are picked and cured before they have ripened. They have very little oil, their flesh is firm and they have a sharp flavor. Black olives, which have been left on the trees and picked when they are fully ripe, are full of oil, have soft flesh and a mellower flavor.

https://www.delightedcooking.com/what-are-black-olives.htm

Pimento is added by human hands to olives. It is a garnish for the olive, a taste enhancer.

All freshly picked olives, no matter how ripe, have a vile, intensely bitter taste. In order to make them palatable, they must be pickled. Since pimentos are sweet and indigenous to the Mediterranean, it's easy to imagine an innovative farmer or chef way back when thinking they would make the perfect neutralizer to the olive's natural acidity. After all, aren't the best dishes created with ingredients most readily available?

Google the string "olive pimento" and I am sure you will garner a large response.