I will trying my first post shading on a F-16. I have seen that I should use a slightly darker shade than base coat and have also seen that I should use a slightly lighter shade than base coat. Which is it? Or is it personal preference? I want this to be very subtle and not overdone.
There are as many ways to do it, as there are artists who do it!
Personally, I start with a very dark grey, almost black, covering everything.
Like so…

Then, I’ll “marble” on some white, mainly within the panels, like so…

Then the color. This is thinned down at least 50/50, if not more. Just light coats to slowly build up the color. Stop when you have the coverage you desire!

I find this technique is great for modern, grey/grey jets. Them suckers get all kinds of dirty and varied in their paint.
Here’s a few examples.
With this F-14, I did go back over some areas/panels with a very thin and lightened mix of the base color.


Before the clear coats…

The usual method has been pre-shading the panel lines with a darker colour, while post shading the center of panels with a lighter colour. Some do use both techniques.
You can also add post shading with a darker colour via a light wash applied with a brush, which is what I did with this 109:

regards,
Jack
Do both. Use a darker shade where you want to show shadows and grime, then use a lighter color where you want to show highlights and fading. This should also be done over the decals. But it all comes down to personal preference.