The undersides are painted on the Tempest. I departed from my usual method a bit though.
I started, as I typically do, with a coat of Tamiya XF-83. On most builds, I stop painting there, and move on to the upper colors, and leave the weathering, fading and shading for last.
On this build I decided to do the shading and fading right after the application of the base coat. For the fading, I simply added white to the XF-83. The shading was XF-66, heavily thinned with alcohol. In the photos it looks a lot more monochrome than it is when you see it in meatspace.
We’ll see how it turns out!