That's a pretty common HSL-style color picker. It's great for artists and designers who think in those terms. Curious, do you have much experience in graphic design, or no?
It makes sense for artists, designers and painters included. This simulation looks like oil paint, and this type of color picker is like mixing pigment to get a hue and then mixing in black and white to change the saturation and value. I guess that makes sense seeing as this is a paint simulation!