Purple leaf

Likely anthocyanins from the cold. Its how the plant protects itself. No need to worry if that's what it turns out to be, I would just get that heater fixed asap. Worst that will happen is they can hermie if in flower, or be stunted if in veg. 65f/18c isn't too cold, I have seen mine as low as 55f/12.7c. I wish you a good grow.

Its just the cold, no need to worry. In fact research is looking into the benefits of these color pigments on our health as its the same pigment found in plums, blueberrys ect and its possible it may even help preserve the cured bud for longer as a type of natural antioxident. If you put the heater back on it will stop, if you turn heater off for the last week of bud you can make your buds look purple ish too like this. Some strains do it more than others and im planning on growing a specific purple strain this year outdoors where I know the temps will dip to 8 or 9 C around harvest at night so it will be a spectacular purple plant. So its up to you if you want to develop the color later near harvest but during veg and the start of flower low temps like this are not the best for growth.

El color morado en las hojas puedo ser por 3 razones, genética, exceso de frío y o falta de fósforo. Por lo que dices y lo que veo en la hoja yo me inclinaría por el frío, si bien la planta muestra que no está cómoda no es demasiado preocupante, trata de aumentar la temperatura dentro de lo posible y aplica algún abono que ayude a la planta soportar mejor las situaciones de estrés como pro-silicate. Dentro de todo no hay que alarmarse, algunos cultivadores buscan estos colores y bajan las temperaturas a propósito avanzada la floración para tener colas más bonitas a la vista y más tricomas, ya que estás situaciones de estrés como el frío sin llegar a ser extremo hacen que la planta se proteja creando más tricomas. Suerte

The plant decided to use a different pigment to capture light, chlorophyll (stuff that makes leaves green) degrades below 67, the pigment for purple is called "Anthocyanin".

It's most likely phosphorous lockout from the cold temps. If you keep your temps up from now on then it shouldn't be much of an issue. I would add a low dose of humic acid on your next feed. It binds to the nutrients and helps deliver them to your roots

I think you answered your own question. Too cold will inhibit certain nutrient uptake and can cause premature fade.