When painting, I usually start with a mid-value low-sat background roughly complementary to the color of the light source. Since I tend towards yellowish light sources for, say, portraits, I typically start with blueish, using that to influence my shadows.
Pixelling, I typically start with black, simply because that's the default background for a new ProMotion canvas. After I've blocked in/silhouetted, i'll change it to a middish low-sat, usually blue-tinted.