Well, cos it’s so full of strong, almost primary colours:
So OK, I have graphemic synaesthesia, which simply means that I see letters and numbers in colour. (I also consciously locate my letters and numbers and units of time in space; my alphabet and my numbers aren’t only coloured, they migrate — down a hillside and into a rabbit-hole in the case of my letters, only to come out again, up a staircase that makes Escheresque right-angle turns in the case of my numbers, and I won’t go into my days and weeks and months and years and seasons right now).
And OK, green isn’t a primary colour (except in when we’re talking about lights). But from an anthropological point of view, red (with white and black) is a basic, basic colour, being the colour of blood; blue is another basic, basic colour, being a variation of black, and being the colour of the sky; and green’s another basic, basic colour (and one of the earliest to be added to language), being the colour of grass and trees and crops.