Actually, I have an idea for what might be going on. It has to do with how the color picker behaves.
The initial location for the color selector is on the far left of the color field, which represents black/dark color. If you just click a color on the horizontal selector the color won't change because that color selector dot is still giving you a very dark version of the color you selected.
This shouldn't affect picking a color chip from the palette or entering a hex code though. Try moving that dot selector to the right in the color field to select a color and see if that works. Let me know if you are still seeing issues.
Owen
Owen Evans
Lead Product Engineer | StoryMaps