Suppose someone points to a vase and says “Look at that marvelous blue—the shape isn't the point.” Or: “Look at the marvelous shape—the color doesn't matter.” Without doubt you will do something different when you act upon these two invitations. But do you always do the same thing when you direct your attention to the color? Imagine various different cases. To indicate a few:
“Is this blue the same as the blue over there? Do you see any difference?”
You are mixing paint and you say “It's hard to get the blue of this sky.”
“It's turning fine, you can already see blue sky again.”
“Look, what different effects these two blues have.” “Do you see the blue book over there? Bring it here.”
“This blue signal-light means. . . .”
“What's this blue called? –is it ‘indigo’?
from Philosophical Investigations, #33