This thread is about 2 years old. lol
But since it got bumped: If a plant is growing fine it's not fusarium... and red isn't that uncommon with plant metabolism when you think about it. Especially in plants that transport nitrogen out of older leaves like happens all the time in the Fall in response to colder temperatures or after a leaf dies. Cannabis even frequently turns purple - which is just a combination of red and blue, so there has to be red moving through the stems at times to make that common coloration. My guess echoes the others who mentioned flavonoids: specifically anthocyanins, but including aurones, chalcones, flavonols and proanthocyanidins. Flavonoids can be elevated due to high levels of UV, cold temperatures, and leaf senescence according to my google-fu.