Since March 20 is the equinox. That leaves March 21 open for the International Day of Colour, sponsored by, you guessed it, the International Colour Association, comprised of national members representing more than 30 countries.
The date was chosen around the equinox because night and day are approximately the same length, symbolically relating to the complementary nature of light and darkness, light and shadow expressed in all human cultures.
If you want to see an interesting and visually arresting website, visit the Color Association of the United States. There you will find that, between color copyright and color forecasting, color is a multi-million dollar business. (What isn’t these days?)