Need some help adding variations

I have a single product that's been selling relatively well so I've ordered some different colours. I'm struggling to add variations to the existing listing. I assume this is because there is no parent ASIN, how do I create a parent ASIN and group child ASINs underneath without affecting the original listing that's selling well?

I've looked through seller university and other discussions but can't seem to find an answer.

Thanks in advance!