The most common type is grapheme-colour synaesthesia where people perceive words, letters and numbers as distinct colours.

BBC: News

The city buses are colour-coded to instantly telegraph what type of route they ply: red buses are express, green buses run short routes between subway stations and other points.

BBC: Living in: Great cities for public transport

This means the robot can distinguish the type of waste based on its colour and drop it into the appropriate bin.

FORBES: ZenRobotics Develops a Garbage Sorting Robot