If depends on a whole load of things, a fluorescent magenta mount would be horrific around a nice traditional watercolour but it must look ok around something, to someone, somewhere, even if just the inner of your double mount.
Everyone has different ideas but some are unwilling to shift from them.
My own are that you can complement, match, contrast or just go totally unrelated just for fun - ic it’s a fun thing you’re framing.
Generally with double (or more) mounts, I would go for darker inner and lighter outer but avoid two dark colours, not least of all
because of the stark white of a bevel between the two mounts, and if the artwork is also dark, two stark white bevels, which of course can be dealt with though.
Some examples, which cannot be wrong because they are right for the owners! A metallic black mount with a silver slip (mount reverse bevelled, of course) giving a spotlight effect, a neutral mount with a dark inner, to contrast against the print’s paper margin and a deep wrapped bevel.
On Bugs, most would go white mount black frame, go to ikea for that
Make your own rules and break them on a regular basis.
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The first P.F.G. (and still one of only two)
outside of North America.
(and mat on mat as I understand). 
