Primer: Tamiya “Fine Surface Primer” spray can
Base coat: A mixture of 90% Tamiya acrylics (XF-59 “Desert Yellow”, XF-57 “Buff”, XF-19 “Sky Grey” in a 4:1:1 ratio) and 10% Tamiya X-22 Clear in Tamiya X-20A thinner (2/3 paint mixture to 1/3 thinner) . For the second coat I lightened it a little bit with a ratio of XF-59:XF-57:XF-19 of 3:2:1.
I did consider adding some kind of texture paint to simulate the non slip coating on parts of the turret and hull of the vehicle. However, the consensus on at least one forum post seemed to be that this would be unnecessary at this scale (a different story for the Challenger 2 I think). So, I was happy to skip this step.
1. Modelling the Challenger 1 and 2 MBT and Variants, Graeme Davidson and Pat Johnston, Osprey Publishing, 2006. (link)