Actually, it's most likely this: When it goes from summer to autumn, to the fall, the leaves all change color. Some turn red, some turn orange... some turn yellow. Pot leaves turn yellow. Your plants are reaching the end of their season, and so the leaves are turning color. Marijuana goes from seed to mature flower bearing adult in one grow season. All that is happening is your plants are basically saying "I'm getting ready to die". Once they flower, they're expecting about 52" of cumulative snowfall to come and kill them to the root.

Edit: However, looking at some of your pictures, you are experiencing death in parts of the leaf. Are you getting any nutrient solution on their leaves? Being six weeks in to flowering, and assuming an 8 week flowering period, you might want to pick up Sledge Hammer from the store and purge your nutrient solution. Feed them straight water till harvest. Something is killing the tips of your leaves. Perhaps take your problem to the "Plant Problems" subsection of the forum. They will be able to accurately tell you what is wrong.