Why not just create a swatch that contains the colors you like?
Good idea. But the problem is when mixing those two colors together using the mixer brush - it then utilizes the millions of colors in the RGB pallet to smoothly blend those 2 colors together.
If the pallet was restricted to 256 for example, then when using the mixer brush to blend, I'm guessing the blend would appear more pixellated because of the restriction on the amount of colors.
To answer my own question however, I tried adding a Posterize adjustment layer on the very top of the layer stack, then reducing the number of levels to somewhere between 10-15 levels.
It seems to be acheiving what I'm looking for, although I was hoping for a native way to restrict the color pallet without having to use adjustment layers
For the mixing brush, if you click on the arrow just to the right of the color swatch in the top tool bar. You will see an option to load solid colors only. Not sure if that will help, but it maybe worth noting...