What dose yellow and green mean?

[limpid cliff]

I was playing Theo town into I saw yellow and green I'm confused dose green mean amazing yellow means bad and blue means fine?

[tiny pollen]

I think it's the voltage
Blue : Low voltage, usable by people directly

Green : High voltage power, need transformer to be converted into low volt and be usable by people

Yellow : Power plant support both low and high voltage output

[limpid cliff]

Thanks