The default "Auto" color is black. If you hit ALT+H,FC [Home Ribbon, Font Color] the first thing you'll hear is, "Automatic, RGB 000," which is black. Unless you change this, you can count on black being what you're going to get.
But, if you want to manually set black (which is all zeros for Red, Green, and Blue) then, after hitting the previously noted sequence, hit M (More) which will throw you into the more color picker, then right arrow from the Standard to the Custom tab. In the Custom Tab, Hit ALT+R [Red], and whatever value is there, change it to zero. Then follow that with ALT+G [Green] make it zero, ALT+B [Blue] make it zero. Then OK your way out. This will set the value to black, with not even the vaguest capability for an automatic change. But the only time I've seen any change from black with automatic set is if you paste in text that's another color.
I'm using NVDA but it does read the colors. If I down arrow after landing on Automatic I land on the first theme color (white) then right arrowing goes across the theme color row. If you down arrow under a given theme color then it reads out the percentage of "intensity" for lack of a better descriptor of that color, working from a 10%, which is either 10% lighter or darker (announced) and each step darkens/lightens (depending on the theme color).
If someone wants me to get into Gradients I will, but please don't make me as I can only describe a gradient as moving from dark to light or light to dark for a given color and getting into all the various top to bottom, bottom to top, diagonal (left or right), etc., gradient configurations could take days.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
One does not discover new lands without consenting to lose sight of the shore for a very long time.
~ André Gide