What kind of coolant should be added to a 1999 Audi A4?

I have a 1999 Audi A4 in which the coolant is starting to dip below the minimum line. I know many coolants are color coded, and that I shouldn't mix different kinds. The current coolant is orange. What kind of coolant should I put into it to top the coolant level off? Should I flush the current coolant and replace it with another kind?|||You mean red- it takes G12, only G12. Top it off with distilled water. Make sure you keep an eye on it, and also make sure the cap for the tank is tightened down.