Go to Settings -> File Types and edit the All Folders type.
Go to the Context Menu tab, then click New and set the Type drop-down to Sub-menu. Name the menu whatever you want and click OK.
Click New again and this time set the Type drop-down to Run an Opus function. Enter Properties SETCOLOR as the function and click OK.
You can move the two new items around in the menu as you like. Just make sure the sub-menu item is directly above the function item, so that the function item is indented in the list.
If there are any other items below the function item then they will initially be indented and part of the sub-menu as well. To move them out of the sub-menu, right-click the first thing below the function and select Decrease Indent to end the sub-menu.
You should end up with something like this screenshot:
