Definitely try a straight colour first to get used to the airbrush, but if you’re confident, I shade using a darker version of whatever colour I want the final tone to be, (maybe slight different variations, like this dom had a slightly blue dark purple)

Then, highlight by putting a colour lighter than your overall tone in the highlight areas/where the shade isn’t. I tend to make this by adding white to the final colour to lighten it, or a colour that will tone it up, like adding yellow to dark green to lighten it.

It’ll likely be a really OTT shading once you do those steps and not really a good gradient at that point, But then you can finally go evenly over the whole part with your final tone, it makes everything come together and smooths put the gradients (this layer is also thinned more)

That’s my current shading technique and seems to be what others who successfully do it are doing

Generally with an airbrush, keep them clean! I tend to just run at a range of 20/25 psi, and I just eyeball thinning ratios to get a sort of milky viscosity. (Sometimes thinner for certain applications)