Hi there! I suggest simply creating a legend image as needed and then create a static (fixed position) hot-spot in SpotEditor and position it in the corner of your 360 product view using the content position controls on the same Edit Spot form.
Here's how the Edit Spot form would look like for the static hot-spot in SpotEditor v3.5 - please see attached image.
PS: you can also use the Text Content tab and create either simple text hot-spot or an html hot-spot using your own mark-up.
Another option is to create a custom indicator graphic (those colored circles usually) and upload it on the Indicator form that you can access by clicking under Indicator graphic on the same form. You can then create a normal (rotating or dynamic) hot-spot and then propagate the current position that you chose for the hot-spot on the first product image to all 360 product images by hitting right-mouse button on your current image in SpotEditor and selecting "Propagate current spot to all" or using the small chain icon with a plus sign under the hot-spot list on the right pane. But this would not be as manageable as the first option above.