[Faraday]

What is blue? No, not the wavelength. The color itself. Exactly what is it?

This question is probably more usefully directed to an expert in human perception, which I am not. But I'll take a crude stab at it, since it arises from the physical world. Blue is name people give to a mental image which we would assign to a category known as color. It arises from the interaction of an photon of a limited range of frequencies with the eye as processed by the brain.

Things can exist as independent entities (matter and energy), as our perceptions, or as pure theoretical constructs. Science uses all three categories, but focuses on what can be known about the first.

While were at it, Bruce, perhaps you could tell me what "number" is. Is it similar to an electron? or "blue"?

[Cultural Jihad]

Whatever we perceive of as blue is really the absence of blue.

As K-9 said in Warrior's Gate:
"The angle of incidence equals the angle of reflection."