I know this was years ago, but I just ran into this problem and solved it. (without having to re-make a product)
I had a store that had 3 variants of a white t-shirt, and they just added the same sizes except now in black. This is what I figured out:
2. Click "Add another option", type "Color" and add your ORIGINAL color, NOT the variant color yet. Click Done.
3. Next, SAVE the product. This is important as Shopify needs to know an edit has been made before it allows you to continue.
4. Next, select the checkbox next to all the variants (there is a box next to "Showing X variants" that selects them all)
5. Then click the "More Actions" dropdown arrow, scroll all the way down, and click "...in another Color"
6. In the box that pops up, you will be prompted to enter a duplicate variant. Type your NEW variant color, along with anything else that applies to the new variant.
For me, I hadn't uploaded any new images at the time of taking these screenshots, so if you have to go into the Product Options and continue editing your products as needed.
I hope this makes sense and helps somebody somewhere!
-Ryan @ Flora Marketing Collective