Like Shagrath said, it is totally fine, I can have that from seedling to just about any stage when I grow. Usually doesn't get worse. If it spreads out and you have leaves dying then its a problem. You will find a lot of discolored stuff throughout the grow like some purple splotches on the branches, sometimes red, yellow light green veins, "rashes", lines etc... as long as plant is standing healthy, and it's not dominating, its all good :)