You see, people are

Program to use Red as stop or no good ,

Yellow to be cautions and Green to be oke or pass or successful or everything good .

Its become social norm and ethical to use this colour for marking something ,

Then come along a person who think differently about colour and like to changed it , then he got attacked from all colour cause it considered blasphemy to change code ethic of something .

While the basic principles is very simple : Colour !