This 6 color pallet consists of like family colors. EX; green yellow and green blue, red yellow and yellow red along with red blue and blue red. When like family colors mixed with each other give a purer secondary colors as the arrows indicate. However when mixed with a non-family color the color is grayed. EX: ultramarine blue mixed with either yellow is a grayed green. This method of mixing continues to react this way all around the pallet.
Not shown is mix the two blues, the two reds and the two yellows together and you now have three new primaries to make additional secondary grays.
The combinations of colors, especially the greens, are unlimited.