SI Codes (Slither.IO Codes) are codes used to chat with other players through skin colors. If you are to use them, write [SI] before your name so people know you are using SI Codes.
A repeating pattern of RED-BLUE means "i am pro". LIGHTGREEN-YELLOW means "i am noob". PURPLE-BLUE means "eat me". YELLOW-STARS means "peace". PURPLE-STARS means "no mercy". BRIGHTYELLOW means you are going to cross the border. DARKGREY-RED means "go easy on me".
You can add three reds before your skin to add a pro-ly meaning to your skin. Three blues means the opposite.