When you say it deteriorates, do you mean that the pigment settles out of the solution?  That's not really a deterioration, that's normal for most paints, and the solution is pretty much as you described for your Citadel paints, as far as using ball bearings goes. They need to be shaken or stirred to distribute the pigments again through the solution.  I use Vallejo's Model Color, as well as Andrea's paints, and they need to be shaken well before use. Some people buy glass beads and put one in the bottle, to serve as an agitator when shaking the bottle. Glass beads are best, since they don't react in the paint. Ball bearings, or BBs, can react in the solution and oxidize.

The same thing goes for enamels. I use Testor and old Model Master enamels, and I stir the paint before use.

Same with my Tamiya acrylics; I shake or stir them, before use.