Like blue, have a soft spot for blue. The best thing about girls putting blue on WeChat avatars is that they like blue and have a soft spot for blue. Blue is a cool color, which has a high frequency in the visible light of electromagnetic waves, with a frequency of 600-700 Hz (corresponding to the wavelength of 500-430 nanometers in the air), and belongs to high-frequency light. Blue is one of the three primary colors and four psychological colors of light, representing the color of the sky.