Why was Glas adapted to mean blue when it's used in so many contexts to obviously mean green? Why was Gwyrdd invented?
The term glas originally meant "green/blue/grey". When the Britons spoke Brythonic they had no need of separate names for them. The only place they saw blue would have been a few flowers (rarely) or the sky - so why differentiate between the colour of the sky and the colour of grass? When the Romans arrived with their coloured dyes there was suddenly a need to tell the difference between green and blue - so the Latin virdis was adopted for "green" (virdis > gwyrdd) (note that Latin V sounded like 'w') and glas took on the meaning of "blue".
To some degree I'd say anglicisation, but also "glas" when refering to shades of green only really applied to natural ideas and transitionary colours such as turquoise(?), and it also carried the idea of freshness i.e the Welsh name for "Freshers week" uses the word "Glas/Gleision" (sorry can't remember the phrase right now). In terms of modern usage many people especially the young use "Glas" for things that are "Blue" in English and "Gwyrdd" for thing that are "Green" in English.
See the notes in 'Hints and Tips' for the 'Colours' section for more info on colour names.
Just added it. Thank you.
Gwair is often used for hay, too, but gwair sych can be used to distinguish that if needs be.
Lovely, informative piece which led me to search the earliest recorded date Britons were dying with woad (which I thought may've predated Roman Britannia) and found it to be Anglo-Saxon. Thank you for pricking my knowledge.
I just did a quick search for welsh words that link to the English word "Dye" in Geiriadur Prifysgol Cymru and it returned lots of words, but the oldest one was "cen", which has a written record going back to the 9th Century (and looks similar in all the other Celtic languages, breton, cornish, irish etc) for:
murex, purple dye made from the juice of the purple-fish; lichen.