welcome to the forum
how many are you painting? what will the mug be used for after it is painted ??
to me, painting blue over red with just one coat often results in an unacceptable color.
since this is on a glazed enamel surface, I can't think of anything except going with a white mug and painting all the colors individually. Again, this depends on how you are using them after being painted.
Photos of your projects may help us to better help you.