[garbanzo]

The color itself. Exactly what is it?

Oh this is simple - it's a perceptual effect.

[MHGinTN]

'Purple' is a subjective designation used to identify a grouping of electromagnetic waves in the spectrum of electgromagnetic frequency; the 'oscillation limits' determine the position on the scale of electromagnetic frequency (I think! ... Physicists, Faraday, longshadow, and others will hopefully correct me here.)