Purple Roots

Last grow season, i cut the base of my plant to go dry it, and left the roots and everything untouched in the pot. A week later i went back, and pulled out my remaning root system. I noticed that parts of the roots were purple. Does anyone know what caused this?

Aynone have a slight idea? what can cause purple tiped roots, and only on a few of them? The weather was never too cold, and i barely added any fertilizer...so i dont think these are the causes. unless im wrong...can anyone help?? any thoughts will help! -thanks-

It could be the gentics. But I might be wrong.