The answer to "how easy is it for the devs?": very easy. The arrows aren't coloured in when drawing, they are pre-drawn in a composite PNG image for all the arrows. The page displays the appropriate part of this image rotated. If they replace that image file on the website, they would have new colours.
The answer to "will Waze change it?": I'm going to guess the answer is almost certainly not. Once upon a time, the segments in WME were different colours. Each "neighbouring" road type (eg minor highway and major highway) was a sharply contrasting colour: so you would find it difficult to to confuse 2 similar segment types. HQ decided to change this to the current "dark-to-light" colour range, meaning that the closest colour to any segment type was the type of segment above/below it. They didn't care much about the complaints then and pretty-well completely ignored the needs of colour-blind users. The last word I heard was a Waze staff member at the Global Champs meetup who basically said "we're doing it, so live with it". Since then, they've done their best to remove contrast and visual cues from the WME UI, so I wouldn't hold my breath.
As for fixing/changing it with a script: I'm not so sure. I am
a script writer, but definitely an amateur one! As it happens, I've wanted to be able to add/replace an image on the website for a long time: the zoom bar my FixUI script re-creates throws errors in the background because the "+" and "-" images it needs are no longer loaded with the editor. So I can't say it's not possible - but it will take someone cleverer than me. If someone points me to a way to do it, I will happily incorporate the option into my script.