Same applies for grey, and many other colours and things. For Indonesian culture, grey is the colour of an overcast/hazy/dusty/ash/pale day, as such, it includes any desaturated colour, especially (most commonly) light blues; so for them two blue banknotes, one saturated and one desaturated are clearly different colours and they are perplexed as to why westerners mix them up. More research of this <https://en.wikipedia.org/wiki/Basic_Color_Terms>. From my research, a lot of this comes from whether a culture's consideration of a colour is from natural phenomena, or from colour theory (mixing the primary colours to generate the secondary colours, developing color wheels, acquiring distinctions for hue, saturation, lightness, etc). Take "orange" in English, or "cokelat" (the colour brown, and chocolate) and "oranye" (to describe the colour of a ripe orange/"jeruk manis") in Indonesia. Sometimes with cross-cultural intermixing, an object could be named after a colour in one culture, then that object is injected into another culture, and that culture then names the colour after that object. Such cultural introductions also extends to mythology and affect, lighter shades could be considered young or easy (as is "muda" in Indonesian), white could be considered for pure or wealthy or sickly or light, dark for ground/earth or peasant or tanned/healthy, red for blood/danger or passion or love; blue-green for nausea or life/vitality/fertility; same also applies for gender and pronouns; man as in mankind, or man as in male, and their inter-cultural/educational corruption/degradations/influences/experiential-biases/subjectivity.

Same with Old and Middle Irish (1200 AD): "glas was a blanket term for colors ranging from green to blue to various shades of gray"

I like to think this may have had something to do with them having both blue and green in their political usage: https://en.wikipedia.org/wiki/St._Patrick%27s_blue