I've been sorting through some large sheets of paper and came across this chart from many years ago. I was always frustrated that a normal mixing chart only showed one or maybe two possible mixes, so I created this one to show 18 possible mixes.
The grid is 27cm x 27cm, created with 9 rows and 9 columns of 3cmx3cm squares. Each square is divided again to create 9 1cm x1cm squares. It's a fiddly to draw up and an even bigger fiddle to paint, but rather interesting. On single square can not really show the full range of possibilities - especially with interesting mixing neutrals like phthalo blue and cadmuim red, which creates Indian red hues, Prussian and indigo hues and a lovely mixed black. (I'd recommend using pyrrol scarlet rather than cadmium red as a warm red though)
It was made with a warm and a cool red, yellow and blue as well as three secondaries - orange, purple and green.
The diagonal line of squares from top left to bottom right just show 9 variations of each of the 9 colours, which were lemon yellow, cadmium yellow deep, perinone orange, cadmium red, crimson red, imperial purple (mixed from PB29+PV19), ultramarine blue, phthalo blue and sap green.
I'd use very different colours today - but this is just one of the many and varied charts I created to test out colour palettes. More mixing charts can be seen here, gouache mixing charts and wheels with a split primary palette here and some sample triads here.