Both colours are best going onto a previously painted surface, but of course this is not always possible. In theory, there shouldn't be a major problem painting either of these colours, especially spraying, unless on small areas, and paricularly compound curves such as spinners. Basically, lay down the first coat as evenly as possible, let it dry, then build up the 'depth' with further thin coats. It's sometimes easier to paint, for example, a matt yellow undercoat, and then follow this with gloss yellow; the final finish can be achieved with the relevant clear coat varnish. The same goes for white and red, the other two 'difficult' colours.
On the Bf109 model I'm currently 'restoring', the yellow wingtips were achieved with two coats of acrylic, sprayed over the enamel base colour green camouflage, allowing the first coat to dry thoroughly before spraying the second.
If you are using acrylic paints, then you'll find it easier to spray onto a base of matt enamel, white in both cases (white or yellow finish).