but when does turquoise start and end and green starts and blue ends? or is there just another color there between them. And then what about that color?

I think you're (accidentally?) hitting on exactly the point there.

For some people's language usage, blue and green are adjacent colors, and thus defining a point that divides them is perfectly fine.

For other people, these are not adjacent -- for some people, there's a single color (aqua? turquoise?) between them, and green and turquoise are adjacent colors, as are turquoise and blue, and it's reasonable to ask about a dividing point between those adjacent pairs.

For those who don't use language this way -- do you consider red and blue adjacent, or do you consider purple (violet?) a necessary intermediate? Are you comfortable defining a point between red and blue, or are you instead comfortable defining a point between red and purple, and a point between purple and blue?

And for all I know, there are people for whom blue and green (or blue and red) have a distance greater than one, or greater than two...