Cold means high blue content. Blue light is the one that most disrupts insects, nocturnal animals, your night vision, your own sleep cycle, and contributes the most to obscuring the stars.
That part is not actually true. Any light will obscure the stars; if you want to compare different colors, it's the middle of the spectrum that contributes most, but the difference is small.
As an amateur astronomer, the LED lights are a big problem because their light pollution is hard to filter out. Sodium is easy, it's nearly monochromatic, so a simple filter will remove most of it. Mercury is not super-hard, the spectrum is not very broad and it's pretty uneven. But LEDs have a broad, even spectrum, which is impossible to filter out.
