Following his advice, I have tried both drybrusing the balls and giving them a light spray of black primer.
The results are below.
Your basic white - for comparison. Prussian guns open up on Dutch infantry.
The 'grey' alternative.
The grey from further away
Personally, I agree with Joseph. The grey / black enhances the 3D effect and seems to be more realistic in the end result. I will now mostly be doing the rest of the batch using Halfords matt black primer.
Thanks for the tip, Jospeh.