I personally like an enamel as a primer for an enamel topcoat. Enamel as a primer bites the plastic a little better, and also gives a better "tooth" for the topcoat.
As far as colors go, that's going to depend on what your topcoat color will be. Typically, lighter primers for light colors. Darker primers will tend toward toning down topcoats slightly. For example, I prime all my armor models flat black prior to spraying the topcoats.
Hope this helps a little.
Gip Winecoff