Materials in Koru are made of layers. Exactly as in your favorite graphics editor, Koru lets you specify some layers and then renders them one on top of another to display the shape on screen.
Besides layers each material also has Main parameters block that contains material-wide parameters. Compared to Owlet the list of main parameters is quite short in Koru:
The Double-sided option controls if the material is visible from both sides of the shape, or just from its front. Here are two similar shapes, where the left one has double-sided material, while the right one has a single-side:
Note that the shapes are the same, but you don’t see the inner part of the right one. That’s because it has single-side material that is visible only from its front.