this is in the strain you are growing. the color is in the genetics as well as it is getting cold.
when your temps are cold in the dark cycle this cold should be more referred as the root zone,
when the root zone gets cold it shocks the plant, in turn you get all kinds of color from purples,
to a blueish tint to even reds some times, so if you dont want
your plants to change, keep the roots warmer
other wise she looks real nice, and her color is normal