Something I noticed whilst revising about group 15 is that the darker the allotrope of phosphorus, the more stable it is.

• white/yellow – very volatile, soluble, pyrophoric
• red – air stable, polymer, more stable, one cross link;
• black – most stable, polymer, maximum amount of cross link.

What's the reason for this? Or rather, why would a more stable allotrope have a darker colour?