The colour blue

Philip and Phoebe, the terrible twins, were having one of their regular philosophical disputes
the other day. It all started because they disagreed over the colour of the sky out of the window.

"Blue. The sky's blue. Everyone knows that, silly," pronounced Philip.

"Well, it looks more like a greeny-grey to me," responded Phoebe. "The sky isn't always blue
you know."

"That's because you're not seeing it properly," said Philip in his maddeningly superior way.
"It really is blue, you know, whether it looks like it or not."

"But that's wrong!" protested Pheobe. "The colour is what you see, not something that's really there. If the sky looks greeny-grey to me, then it's greeny-grey, surely? How can it be 'really'
blue?"

"Oh, that's because you don't understand about Science," responded Philip airily. "Scientists
say that 'blue' is light with a wavelength of 0.000465 millimetres, so if that light doesn't
look blue to you, you're just wrong."

* * *

Who is right? Is 'blue' really there in the world, whether it looks blue to you or not?
Or is something only 'blue' if it looks blue to you?

When you have thought about this problem, click here.