Temperature

15C is too cold even when lights are off.. that will potentially impact cell biology/chemistry. you need to do a little trial and error to learn what intensity you need to provide to prevent stretching... too much light will stunt the plant and be just as bad of an outcome. if those new nodes stack on top of each other, it's too close / too strong (dimming or raising light accomplishes same thing for the most part). A seedling may even fall over if the light is too strong... trying to put more distance between it and the overly powerful light relative to stage of life cycle. anyway, the coloration in the blurry pictures is probably caused by the 15C environment. You'll see "autumn" colors in cold weather because the plant isn't able to process things properly at those temperatures.