That Time Magazine link (in the OP) to an article that has a brief explanation about why the artists chose that grayish hue.

“Bold colors and patterns are a constant in Sherald’s work — from the bright red fascinator adorned by the young Miss Everything to the multi-colored print dress that pops from the canvas of her 2017 work, Untitled — but the skin of her African American subjects is often presented in varying shades of gray. She has also been said to seek out subjects who she sees as “existing in the past, present, and future simultaneously,” according to Artnet.”