For the color variants you have, you can either let the theme's coding provide a color for you, or you can upload an image that represents that color.
We recommend using images that are 100x100px, and keeping them the same to ensure the aspect ratio is the same.
For example, if you have a color called 'Déjà Vu Blue', then name your image
Other example, if your color is 'Blue/Gray', then name your image
Most simple example, if your color is 'Black', the name your image black.png.
On the Edit HTML/CSS page, locate and click the Assets folder to reveal its content.
Under the Assets heading, click the Add a new asset link.
Upload your image.
Repeat steps 2 and 3 until you have uploaded all your images.
There is also this Shopify help guide: