This is NOT a grayscale issue. Whatever color I choose first will not be anything else except that color. I momentarily used white for a marking, and now I cannot color with ANY other color except white. Neither black nor a test hue to see if it was simply a BW swatch issue like how MS Paint has.
I am using a base, however, I don't believe it is due to them, as those submitting photos have been able to place down markings just fine. This is an Adobe issue.
(In the example, I select a color but it continues to color white despite clearly not)