You can add color swatches to your product and collection pages using these settings found in Theme settings > Product Tiles
You can enable color swatches for filtering on collection pages using the settings found in the Collection template > Product grid > Enable color swatches
How does it work?
Once you have enabled the color swatch settings, your theme will look for any variant category with the name "Color" or "Colour".
To change the word to something other than "Color/Colour" you can do so in your languages area under Edit theme content > Products > General > Color swatch trigger.
Note: If you're using a theme trial in your online store, Shopify restricts access to your language settings until a theme purchase has been made. For testing, we recommend using the default 'Color' variant name.
The theme will then look at the variant names you've used (in this case Heather grey and Green) and look for either an HTML color name with the exact same name or a .png image file in Content > Files with a filename-friendly version of your variant name.
Naming Your Swatch Files
Green swatches would need to be named green.png (capitals become lowercase)
Heather grey swatches would need to be named heather-grey.png (capitals become lowercase, spaces and other special characters become hyphens)
.jpg files will not be shown, you must use .png