Always use the Brush.color setting in the Draw mode side bar.
Technically the "Color" panel in the sidebar can draw in other modes,
but it's polling for explicit tools that limit it to the Draw mode
implicitly.
Use the appropriate brush or unified color in other GP modes based on
the unified paint settings flag.
Worth noting this was technically broken for a long time:
The default was that unified paint is disabled, so the GP panels would
use the brush color as expected. Enabling unified paint would then break
the colors, probably wasn't done very frequently since GP files
and e.g. mesh sculpting don't mix often. The default was changed in
#129127 to enable unified paint by default, which now breaks GP colors
by default.
Pull Request: https://projects.blender.org/blender/blender/pulls/129790
449e7229f3