Cannabis's natural color is actually purple. Chlorophyll is actually what makes it green. It's only when chlorophyll is in colder climates is when the purple comes out because green no longer the dominant color. If you also notice, the growth of the plant slows when it's below 64. This is because chlorophyll production slows as well (almost seizing the chlorophyll's ability to perform photosynthesis). It's not bad per say, but if your looking for fast vegetative growth compared to fast floral growth, I suggest higher temps.