Yellowish leaves/Slow growth

The plant looks perfectly healthy to me! (maybe add a bit more Nitrogen (N)) to help the yellowing, and if the purpling stems gets more severe then add more Phosphorous (P) into your soil. As for the slow growth the low temperature may have caused it! Cannabis growth temperatures range between 22-28 depending on the variety (strain), some unique or specially bred varieties have stronger cold hardiness and can withstand temperatures as low as 18 degrees Celsius daytime (and even lower!!). Seems like your plant would like a higher temperature (which is usually recommended to increase the photosynthesis rate) temperature, try increasing your grow space temps to about 24-26 degrees by putting a heater near your intake vent. Hope this helps! Good luck and happy growing :)