what if you want to say something was blue but the other person takes it as you saying green? since kastra is blue or green?
I guess, valyrians just do not distinguish these colors. For example, english speakers also count blue and deep blue as one color instead of russian speakers (because russian has two different words for blue and deep blue).
Also, as far as I can tell, there are some natural languages, which have one word for blue and green.
Yes one of them is Japanese (nowadays we have 'midori' = 'green' and 'aoi' = 'blue' but historically 'midori' used to mean both).
As for the discussion, it simply wouldn't take place at all for HV native speakers since they'd would not see the difference or apply a noun but it'd still be quite futile.
"Sky greenblue?" "No, grass greenblue". "What's the difference again?" "No clue."