The NTAG I2C Plus chip will output power from it’s energy harvesting pins at all cost, ignoring the NFC communication completely. If you put a red LED across it without any current limiting resistor, because the output voltage is 3.1V it will overdrive the LED to such a degree that the chip will brown out.
I have tried some purple LEDs. I had them working on the early versions of the Apex Mega spectrum, but we abandoned that in favor of the auto cycling RGB LED. We made samples of purple xLEDs, but their performance was very poor, and will likely be absorbed very effectively by the skin, so we abandoned it in favor of other priorities. Some high brightness purple LEDs can reach 7V forward voltage, but even the 3.5V ones only light very dimly from an NTI2C+ because it can only harvest up to 3.1V and that is very much on the lower end of their output