Tigger isn't sure either. I've added corners (for want of a better term) to my Hollow cubes . I have a hard time with the uneven edges so I decided to even it up and charcoal seemed most appropriate. I'm not sure if I will add any more borders to it. If I do it will probably be a jewel tone...maybe red. I'm still fascinated with the 3D effect. What would smaller cubes in the corners and borders of charcoal do to it?