What is the best colour primer to use?

This will depend on what it is you are painting and the colour of the top coat.

Never forget that the colour of the primer will have an impact on the colour of the top coat. If you were to paint two of the same items with the same top coat but one undercoated with a white primer and the other with a black primer, the results will be very different.

For example, if you were to paint a figure that is going to have a yellow shirt or any bright colour for that matter, I would suggest a white primer as this will give you the brightness of the yellow. If you use a darker primer, the yellow will appear darker and "dirty".

If you were going to paint something that does not need to have such a bright colour, then I would go for a grey primer. You could use a black primer instead of grey and get a slightly darker finish.

I tend to use black on figures that I intend to drybrush. The black which will create shadows on the recessed parts of the figure.