The selected thumbnails of the moulding/matboard can be saved to their associated SKUs when you save the project.

You can also save these SKU thumbnails separately.

Click File > Save SKU Images to save the moulding/matboard thumbnails selected to the appropriate SKUs. 

Select the checkbox for each new texture and/or color selection that you wish to save with the SKU that has been entered.

The checkbox is grayed out for any SKUs that already have a texture assigned to it.

Click the Save button.

The next time you select this SKU when designing a project, the saved texture/color will be applied to the design.