I think it can often depend on the item. I have a mix of variation and single listings, and I tend to choose one or the other based on whether I can make a decent gallery image that shows what the item is and can indicate there are further options, without making the image look messy or confusing, since it's the first thing people see when they're looking for something. (As an example, I once had a listing for a necklace where people could mix and match design options, providing 30 different variations in total, none of which I could provide images for in eBay's gallery because you can only show one variation and not the result of two choices).
For a ring, I'd use a variation listing, because only colour is a visual difference (i.e. you wouldn't also need to show people what it looks like in a different size).
I haven't tried to do this for a while but I know you used to be able to create a variation listing for an item and only have one variation available - I used to do that when I knew I'd be adding variations but didn't have them just yet. That way I could start selling the single item and not have to create a new listing when the other stock came in. If you can still do that, you can test out whether single or variation works on a couple of items, then you can either leave as is for single items, or if you want to use the variation format you can just add other items into a pre-existing listing rather than ending everything and starting again.
Edited to add: just be aware of eBay's duplicate listing policies - it's often better and safer to do a single variation listing for same items if there's only one difference between them.
