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.

The purple color in the leaves can be due to 3 reasons: genetics, excess cold, and/or lack of phosphorus. From what you say and what I see in the leaf, I would lean towards the cold. While the plant shows that it's not comfortable, it's not too concerning. Try to raise the temperature as much as possible and apply some fertilizer that helps the plant better withstand stressful situations, like pro-silicate. Overall, there's no need to panic. Some growers actually seek out these colors and intentionally lower the temperatures during late flowering to have prettier buds and more trichomes, as these stress situations like cold, without being extreme, cause the plant to protect itself by creating more trichomes. Good luck!

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.