My grid for my 12 pencil set. I've labelled the axis with numbers and letters. Just to make it easier to locate the colours used. The diagonal edge is the whole colours. I used the same pressure on both colours to get an even mix. 

Column 1 and row a - pure yellow

Column 2 and row a - light orange

Column 2 and row b - pure orange

Column 3 and row a - mid orange

Column 3 and row b - dark orange

Column 3 and row c - pure red 

Etc

The quarter circles are similar mix comparisons. They can also be used to compare different %mixes of the same colour mixes.The T's are 3 colour mixes and this could be added to to make a 4 colour mix. I would do these during a drawing.

Interesting that 6c in the bottom brown quarters should be pure purple. With these pencils it's made with light and dark blue, and pink, as shown in the top T. 

As this is a useful tool, I'm going to do a neat one in it's own notebook, so I have space to do the 4 comparisons and T combos. Just so that I don't repeat myself each time I want to mix colours.