SOLIDWORKS 2018 provides a variety of new enhancements. One of the new enhancements is the ability to include the material and the visual appearance of the parent part in the derived part when using the SOLIDWORKS Mirror Part command (Insert -> Mirror Part). If you see that the Mirror Part command is greyed out that is because you should pre-select the mirror face or plane.
The Material checkbox in the Mirror Components PropertyManager is selected by default and lets you propagate the material of the mirrored part from the parent part. Also, the Propagate from original part option in the Visual Properties section can be used to transfer colour assignments from the parent part to the derived part.
The Material and Propagate from original part options are available only when you mirror a part or when you insert a part into another part for the first time. These options are not available when you edit the mirror feature or the inserted part. Also, the material and visual properties of the derived part are not linked to the parent so if you change those properties for the parent part, the derived part doesn’t change.