The colors refer to variations in wavelength (light ~ sound) adjusted to human perception of soundwaves (https://tinyurl.com/3bzy7a7s). It helps muffle ambient sound that might distract your brain. Studies show they have effects: brown = executive function/memory, pink = sleep/concentration, green = calming, white = productivity/annoying.

Dark noise isn't an accepted term, it's a marketing term for a tweaked version of brown noise