Fix #130651: Tint tool does not show correct color in header

The issue was that the UI code still used the unified paint settings.
It was decided that in `PAINT_GREASE_PENCIL` mode we don't use
the unified paint settings and always use the settings on the brush.

The fix uses the `brush` as the owner of the `color` property instead
of the unified paint settings.

Pull Request: https://projects.blender.org/blender/blender/pulls/131452