Glas can mean grey-green or even plain green, as well as grey. This is most obvious in some placenames. Glas is green in both modern and old Irish which is how it probably got into the placenames.
Gaelic also views colours from a different frame of reference than English. Gaelic colours centre around blue-green and red-brown. I think other languages/cultures have similar (but different) frames of reference.
@athairbirb @Magess @fitheach I love that graphic. Sadly Irish is taught in schools over here with a fairly straight correspondence to English colours, which is a huge shame IMO. With the exception of "Rua"/"Dearg", but even then I was only told that "Rua is for hair, dearg for anything else" which is just false.
My kids are still being taught the "glas for green grass, gorm for blue skies or sea, liath for pencil-grey", which is a parched version of the language.
@seachaint much the same in GME. It's quite an old 'problem' too, I recall being taught English colours with Gaelic words (if that makes sense) and not the nuances of Gàidhlig colouration.
I didn't get to take art 'anns a' Ghàidhlig' in secondary school, so missed out on learning about it there too. Something I think should be used in modern art classes in Scotland and Ireland (and areas with interest)