The whole plant is red?

I came across some ditch weed this last summer that was growing wild and the entire plant was a deep red, burghundy, blood like color. There were several like this but also several nice and green. It had the characteristics of a sativa.

I was just wondering why it was red? It doesn't really matter. I just thought of it and am just curious.

It was located in the northern midwest and it was in mid september. Wish i had take some pics.

Thanks

google pheno. they determine the subtle differences that are in each strain. late