I washed some fleece - haven't a clue what it is, I had it given, and they didn't know either - and then tried to dye it brown. Not being very good at mixing colours I asked my knowledgeable (?) husband who said "oh, it's easy, you just mix all the primary colours together".
This is what came out :-
Gorgeous colours, but not brown!
Anyway, I spun it and then consulted an artist friend who advised me to add yellow/orange. Mine was actually orange/red, but it worked and I ended up with a slightly reddish dark brown.
The next colour (green) was a lot easier to mix. I would have liked it a bit yellower, but ran out of that colour. It actually looks a bit bluer here, drying in the sun, than it really is, but it is in full sunlight and I don't think that helps.
The last colour should be a lot easier as it's blue. I don't think even I can get that wrong!
. . . but don't hold your breath!!