By default, in Shopify collections, you can only display the main products and not their variants. With the Iris theme, you can easily show Color and Size variants by simply ticking the checkbox.

Open your theme customizer, select a collection template, and then select the Product Grid section. Within that section find the Product Card > Show product variants in the collection checkbox to enable this feature.

Important: make sure that all your product variants have their images set. Otherwise, the product card will be displayed with a white background instead of an image.