Glas refers to the whole of the colour spectrum that English divides between green and grey. It means "Grey and green", i.e. it is always referring to both these colours. Just as English refers to the objectively distinct colours of wine and blood with the single word 'red'.
Similarly 'Donn' in Irish includes brown and burnt oranges. It is best not to think of colour terms as translating English words, but as different partitions of the colour spectrum.
Irish has a lot of colour words that don't translate into English - and it has different words for the same colours contingent upon whether the colour is natural or unnatural. So, two words for green - one referring to eyes, or grass, or sky, the other one referring to coats, or boxes, or cars.
glas - green, grey
uaine - green
For now I'm sticking with 'glas', because people will understand it. But as I get more confident and competent I'm going to take some time to study the more subtle side of colloquialisms etc.
It's not cognate with glaucous believe it or not. Glaucous is derived from a term for a type of fish in Homeric Greek, which is reconstructed to derive from an otherwise absent Indo-European root, in fact some scholars think it isn't Indo-European at all.
Irish "glas" is from Proto-Celtic glastos, which comes from a separate Indo-European root (exactly which root is contested, but none are like the root for glaucous).
What do you mean by:
Glas and also gorm can mean "the natural colour appertaining to a thing"
I've never heard them with that meaning, are you referring to uaithne being used for artificial greens?
An bhratach trí dhath .i. uaine, bán, agus flannbhuí, an suaitheantas náisiúnta.
That's about the only place that you'll see flannbhuí written, and very few people have read Bunreacht na hÉireann as Gaeilge.
Dineen, for example, didn't have an entry for flannbhuí and lists "orange" under órdha, which is also "golden".
Since you seem to know so much, I've been wondering what the pronunciations would be for "mh" and "bh" when they're part of two mashed together Irish words. (I'm speaking, of course, of the change dependant on vowel construction, and not the exclusive "v" sound in some dialects.)
As the mh in comh is silent, the pronunciation of bh is unaffected, and is variously "w" or "v" depending on dialect and other circumstances.
I re-read my comment and realize that I didn't make myself completely clear. I mean specifically words like flannbhuí or Lámhfhada. (When I was reading about Lugh Lámhfhada is when I realized I had no idea what the rule would be.)
Edit: since I can't reply to your answer I'll do it here. Thank you. You're a huge help. Have a lingot and an upvote.
The pronunciation of each part of a compound word isn't directly affected by the other word - you pronounce each part as though the word wasn't a compound. The way that stress is applied to the compound word might be a bit different, and in real life, you might sometimes get some interaction between certain sounds due to the mechanics of speech, or a particular speakers habits, but the constituent parts of a compound word are still recognizable as individual words.