The string is refering to itself. "This" i.e. this string, "is not a traffic light color." - this is to alert the traffic light not to attempt to turn on the color described by the string, because the string "This is not a traffic light color" does not describe uniquely a traffic light color, and the result may be unexpected, and perhaps even detrimental to the traffic light.

Your "symbols" is incorrectly defined.

This is what I did yay

wdym pink is not a traffic light color

The dictionary method needs to instantiate a "useless" dictionary. This one does not.

The dictionary method isn't hard to read though, as long as you know what a dictionary is. Also, isn't it faster?