Hi Mary,
This is tricky. There's no definitive answer (that I can give you). In Irish, colours are interpreted differently than they are in English.
I'll try and explain it as I understand it.
In the simplest terms glas usually applies to natural greens - grass, vegetation etc. and also related eco terms (the Green Party, green as in eco-friendly etc.), or green as in inexperienced, young etc. But uaithne may also be applied to verdure. depending on the shade of green in question.
Uaithne/Uaine usually applies to vivid greens, and dyed or artificial greens. But it's not as straight cut as that as glas can be and is used in these situations too and some people will just use glas for all greens. I don't know how this plays out accross dialects or if it's just a school thing or what.
Where it can get confusing is glas very often means grey (I would assume grey before green if I heard glas) - cloch glas is a grey stone, madadh glas is a grey dog, spéir ghlas is grey sky, carr glas is a grey car but cnoc glas is a green hill and féar glas is green grass. I'd really only use liath for grey where it's a light grey or almost white colour, or a person's hair for example. Not for a darkish grey, and usually not for greys in the natural world - animals, the sea, the sky and so on. I might also use glas for dark, dyed greens but uaine for the green in the Irish flag, for example.
If I wrote éadach glas I'd mean grey clothes even though some people might understand that to mean green clothes. If I wanted to say green clothes, I'd use éadach uaithne. If the clothes were a very dark green though, I might use eadach glas and not distinguish between English grey and English green at all.
So I guess with glas I think of it as applying mostly to the natural world, shades of green or grey (and sometimes blue) on the darkish end of the spectrum, with shades of English greens and English greys and some English blues being shades of the Irish glas. Uaithne on the other hand I'd apply where dyed or bright greens, or vivid, lush greens, are in question.
While looking for a way to explain myself, I found these blog posts on glas vs uaine. Hope they help.
http://nimill.blogspot.ie/2010/10/gorm-glas-uaine.html
http://blogs.transparent.com/irish/beoir-uaine-no-glas-no-ceachtar-beer-greenuaine-or-greenglas-or-neither/
http://blogs.transparent.com/irish/beigil-uaine-no-glas-which-type-of-green-for-bagels/
http://blogs.transparent.com/irish/aibhneacha-glas-no-uaine-rivers-greenglas-or-greenuaine/
http://blogs.transparent.com/irish/more-green-ideas-mostly-not-colorless-sorry-chomsky/
Dea-ghuí,
Barra