"There is no Welsh word for blue"
I know QI once falsely made this claim, though we know that "blue" in Welsh is "glas". However, "glas" is listed in some dictionaries (including Bangor's under green as well as blue. Is glass still used for both colours? And if so, are Welsh people calling blue green or green blue?
Many languages don't distinguish between blue and green. https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction_in_language#Celtic
The Welsh word glas is usually translated as "blue"; however, it can also refer, variously, to the color of the sea, of grass, or of silver. The word gwyrdd (a borrowing from Latin viridis) is the standard translation for "green". In traditional Welsh (and related languages), glas could refer to certain shades of green and grey as well as blue, and llwyd could refer to various shades of grey and brown; however, modern Welsh is tending toward the 11-color Western scheme, restricting glas to blue and using gwyrdd for green, llwyd for grey and brown for brown.
Colour systems vary a great deal in different languages. glas has several meanings in Welsh apart from 'blue', including 'fresh' - gleision is used for 'freshers' or 'newbies'. It is used to mean a 'fresh green' for things such as leaves and grass, for example, and also for 'clear' as in awyr (g)las (a clear blue sky).
For a generic green, though, gwyrdd is the usual word.
brown is also relatively new in the last couple of hundred years. Earlier, more specific alternatives include cochddu (a dark brown), llwyd (for a dull or greyish brown) and gwinau (chestnut brown, or 'bay' as in a bay horse).
melyngoch is an older word for 'orange' and also for 'auburn'. melyn can be used for a cream or very pale brown colour, llwyd-felyn for beige or khaki. cochlas for purple.
Interesting, could you give an example sentence for "glas" meaning "fresh green"? This might be a stupid question, but what is it about the phrase "awyr glas" that makes "glas" mean "clear" instead of "blue"?
- Mae ceffylau'n pori'r borfa las. - The horses are grazing the fresh pasture.
- y gleision - the freshers (new students at university, for example)
- glasu - to begin to grow (of crops, etc) (and many other meanings)
In awyr (g)las it is used to mean a 'clear sky' or 'blue sky' or both together.
Have a browse in any good dictionary for more meanings and uses of glas.
In short, glas is both blue and green. But if you want to be specific, another word for green is gwyrdd.
Glas can also be used for shades of grey (or llwydd), which makes it rather versatile;
Check out the video "The Invention of Blue" by the "vsauce2" channel on YouTube. It does a really great job explaining why you run into this weirdness with blue and green (and sometimes with gray, too) in many different languages.
tl;dr - Blue is a rare color in nature, and for much of human language and history what we call "blue" was lumped in with green. It wasn't until the color blue became more widespread (lapis lazuli trade, indigo exports, synthetic dyes, etc) that it became widely perceived as a distinct color.
To me, "glas" is blue and "gwyrdd" is green.
I know in Japanese the word "ao" was used to mean both blue and green, but more recently the word "midori" has come into use for green.
My Japanese teacher told me that the older generation still mostly only use "ao" for both. The grass is bluer on the other side :)