A lot of painters have difficulties painting bright reds and yellows over a black basecoat. Both colours tend to have a weak pigment regardless of the paint manufacturer. This means you will need a lot of layers for an opaque coat, which is both time consuming and prone to clogging up details.

This is what I do: Start with a suitable Foundation Colour which is brighter than the Colour you want to achieve. For reds I use Macharius Solar Orange, for yellow Dheneb Stone. Both shouldn't require more than 2 or 3 thinned coats to cover over black. Next you choose your colours, say Blood Red and Sunburst Yellow (in fact I recommend the Vallejo Game Colour equivalent because they've got a stronger pigment than Citadel) and instead of thinning them with water, use the appropriate Vallejo Game Colour Ink (Red and Yellow Ink). This way you improve the paint flow without sacrifing the "strength" of the colours like you would thinning with water. Also the tinting effect inks have over bright colours aids the pigment of the paint.
With this mix, two coats should suffice for the red, the yellow might need more.

All in all this technique usually requires four or five layers for a vibrant result as opposed to the eight or ten thin coats you would usually do.