To Remove a Material Assignment from an Element(s)

Select the Remove Material tool ( Visualization > Home > Materials > Remove .
In the tool settings window, click the Remove Assignment icon.
Select an example of the element from which you want material assignments removed.
In the Apply Material tool settings, the Palette and the Material display in their respective fields.

All elements with the same attributes for level and color will be affected.
Enter a data point to accept the removal.

Removing a material assignment from an element(s) (Material gets removed from all elements with the same level and same color)