Can’t say for sure, because there is no model to look at.
But this can happen when using Groups and Components… And in depends on how an object was originally painted.
Objects painted from Inside of the Groups ‘Edit Mode’… can’t be overwritten by trying to paint them again from Outside of the Group.
So, you may need to go back into the edit mode to change colors from within. OR, Reset the color to the SU Default, to have the option of being able to paint from outside of the group once again.
Things can get a little tricky if there are some nested groups and components in play.
If this doesn’t seem to work fairly easily, then please upload your model so it can be looked at more closely.