Don't panic when you have the lone red zit. Apply oil-free foundation as you normally would, and then add a dry, oil-free concealer to areas that need a little bit of extra coverage. The most common mistake I see women make is that they use the same concealer that they would use in their under-eye on their zits. This is by far one of the worst things you can do.
Under-eye concealers are creamy and luminous and almost always filled with oil. By using that on your zit, you are basically pouring more oil into the area, which will only cause more breakouts and worsen what's already there. I cannot live without Laura Mercier Secret Camouflage ($35) in my life, for myself and my clients. It is oil-free and will cover everything including a tattoo, so if you have this in your possession, you will never have to worry again.
