I did a fair amount of searching about and kept coming back to the colour wheel.
Color wheel - Wikipedia, the free encyclopedia
The colour coat I want to apply is a tertiary colour - brown.
If you look at this link here =>
How to Mix Paint Colors to Make Brown: 9 Steps (with Pictures)
It shows which components of the colour can be used to make brown - on the same site there are five other methods of doing this =>
5 Ways to Make Brown from Primary Colors - wikiHow
So of those methods I determined, by thinning the paint I have, that Manganese Brown 480 is a purple and yellow based brown.
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Now that I know what has been used to make up the colour I've got I was thinking I could use the information in this article =>
Undertones in Color
To narrow down the likely under colours that would work for the brown I've got.
This guy (sorry I assume a male writer) reckons that all non primary and secondary colours are in effect "grays" - even though they are not gray (if you see what I mean?). So to work out what colour you want to extenuate you need to use the opposite colour of your mix as the under colour.
For my purple and yellow based brown I was going to try a purple under colour and then a yellow under colour to see if this indeed works as this chap says.
A yellow under colour should produce a darker more purple brown
A purple under colour should make a more yellow / gold like brown
I wanted to test this out - I might do so later on - but at the moment I've got better things to be getting on with!