Siblings metafields
Feature introduced in Pipeline 6.2
There are two product metafields definitions required to connect each of the products together and generate linked swatches.
Use the metafields editor under Settings in the Shopify Admin:

Both metafields will be defined under the category of Products:

Choose the Add definition button. If this is your first time creating a metafield, the options may look similar to this, simply click on one of the Add definition buttons:

Tip: If you are new to creating custom metafields, a good step-by-step walkthrough is also available in the Cart Upselling article.
Next, create these two metafields:
Last modified 3d ago