Can I get a tattoo over a scar?

Usually, Yes.
But it depends on the size of the scar. I can't tell you if it is possible or not without actually seeing the scar. Ask a tattooartist in your neighborhood if he could take a look at it and what he thinks of it.

Is it permanent? What about the colors?

Yes it is, but every tattoo will age. It depends on your skin, the placement, how often it is exposed to the sun, and how you take care of it in the healing process and so on. If you have it once you will have it for your entire life. Unwanted or older bad looking tattoos can be covered with a new one or be removed with a laser, but bare in mind, removing hurts more than doing the tattoo itself!