For instance, this is the appearance of color variant Brown by default
If you want Brown to be lighter or darker or different color which is similar to the appearance of your real products.
You can replace or create custom colors by going to Settings > Add color
Enter the name of the color that you would like to custom > Select a color
Result: the color of variant changed
Another case is your product has a color variant whose name is special and not defined by any colors in default setting. The undefined color will appear as white on product options. That's not good anyway.
So you need to modify these undefined colors either. For example: Denim Blue, Rose Red,..etc