By default, in Shopify collections, you can only display the main products and not their variants. With the Flux 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: if a product variant doesn’t have an image set the main featured image of the product will be used instead.