The leaves look like they're purple because of "sun tan", which is essentially when the light "tans" the foliage. Notice I say
tan not
burn. This isn't necessarily a bad thing, its just a very common occurrence with artificial lighting. The plants that I have growing under my 300w LED are about 15-20 inches away from the light. I say 15-20 inches because some of my plants are taller than each other. Sometimes the light tans the veins and stems the same purplish color, it just happens.
As for the stunted growth, it seems like you've confused the seedlings in a way. If you're going to run the lights for 24 hours, do it for two weeks instead of 3 days. Also, 10 inches away from the seedling with a 150w LED seems like a lot for such a young plant. Suspend the light farther away from the plants and let them bounce back.
In conclusion, I think your purple seedlings are confused right now and aren't sure what to do because they aren't in consistent growing conditions. It sounds like the intensity of the light has tanned the foliage and made it turn purple, as well as the light being too intense. Just don't baby them too much and keep the plants on a consistent 18/6 light cycle. 18 hours on, 6 hours off. Keep the light suspended about 16-18 inches away from the seedlings and let them grow up towards the light. As the plants get more mature, you could probably get away with 12-15 inches away from the plants. But NEVER keep the LED below 10 inches. LED's are briiiiiiight!