Anybody who says there's a universal rule for "muting" colors, and thereby enhancing the "scale effect," is fibbing. There just is no such rule.
A good way to start experimenting, though, would be (generally speaking) to add some medium grey to the base color and see what happens. (Exception: never add anything containing black to a yellow base - unless your objective is a nauseus shade of green.) Do a test run on an old kit or a piece of scrap; let the paint dry and see how you like it. Don't feel obligated to limit yourself to grey. A "black" hull, for instance, can profit from the addition of a tiny bit of blue.
The one near-universal rule I'd suggest is: if the observer is conscious of the "scale effect," it's probably overdone. The target should be subtlety.
This sort of thing is fascinating; it can push model building into another realm. Good luck.
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