I use the LAB color picker most of the time. The range of L is between 0 and 100, and the ranges of a and b are both between -128 and 127. This give us a total 101 * 256 * 256 = 6,553,600 colors. Although a and b are theoretically unbounded, I believe ArtStudio Pro is using the CIE 1976 (L*, a*, b*) color space standard, which purposefully fits in the sRGB color space that has 256^3 = 16,777,216 colors.
It seems that regardless of how we pick colors (LAB, HSV, CMYK, etc.), they're eventually converted to RGB mode, which fits well with the 8 bit sRGB color space.
I'm curious what happens if we create a painting from scratch of P3 color space. We know P3 has 25% more colors than sRGB. Does it use a 10 bit color depth? Is there a way to pick a color, say a more saturated red than the most saturated red in sRGB, from the color picker (either wheel or slider)? Right now I see that the slider value is still bound to 256 for RGB and LAB, so I assume #ff0000 from the color picker is the same color in P3 and sRGB.