there are lots of other differences also especially in personality, size and behavior...
the wheels turn inside the head of a greenwing...all the time....but many times they are sedate in behavior..
the scarlet is in motion, much like a windup toy, and they are ALOT more emotional..very full of pride and you can't talk them into things like a greenwing.
I think scarlets are pinchers...
greenwings are chompers...
lol just two other differences.. oh yeah, did i mention size?
and color. If its ruby red, its a greenwing..if its fire-engine ferrari red its a scarlet....
OH THE TAIL...did i mention if the tail is longer than the body its a scarlet?
greenwings have short stubby tails in comparison..not that you could tell the difference with my two sitting there (tails are alllll gone thanks to roughing it with each other )

lol, just thought someone would like a few observations from living with the two funny red birds.

I don't know about the short stubby tail on GWs. Mine has a long tail. Its longer than his body. I think the biggest difference in a GW and a scarlet are the facial features. A scarlet has no feathers on their facial patch and a GW does. Another huge difference is the colors. Scarlets have the primary colors (red, blue and yellow) while GWs have green on their wings (as their name implies) and blue on the tail.

I also think greenwings often seem to have larger beaks/upper mandibles than the scarlets. And as someone said, they have different shades of red. I think scarlets - especially in good light - seem to have a more "yellowish" red, especially behind the eyes.

I always recognize Scarlets by the lack of "war paint" and that their yellow feathers look a little like bananas XD
The colour difference can be subtle when you aren't used to seeing either though for sure, both are big, red parrots

Wow they are both beautiful birds. But I find the Scarlet to be more striking in color, or maybe it was just because he was out in the wild it looks like ?