Nyx

Rulings

A player’s devotion to a color is equal to the number of mana symbols of that color among the mana costs of permanents that player controls. — Nyx’s first ability will apply to nontoken creatures from the moment they enter the battlefield. Notably, a nontoken creature entering the battlefield will cause the constellation ability to trigger. — The chaos ability isn’t a mana ability. It uses the stack and can be responded to. You choose the color as the ability resolves. Once the ability starts resolving and you’ve chosen a color, it’s too late for anyone to respond and change your devotion to any color.