When a product is digitized (i.e. rendered into 2D and 3D), it gets separated into product elements. Elements correspond to (groups of) pattern pieces. In the BRIKL dashboard, you can view and assign a material to a product element.
You can find the Element Tab once you've selected a product from the Catalog. Make sure to assign a material to each product element. Since materials are linked to colors, the corresponding colors will be available in the 3D Configurator for the elements you have selected.
If a product element does not have any materials assigned to it, there will be no colors available in the 3D Configurator.