Steve

I agree with you about Indigo, but the (purely abitary) namings of the colours in the visible spectrum which were decided by no less a figure than Isaac Newton still seem to be used. You do get quite some variations in colours between spectra produced by prisms and diffraction gratings as many types of glass absorb more light towards the violet end of the spectrum - sometimes quite selectively. Perhaps Indigo was a result of the composition of the prism that the great man used.

There is a fairly good spectrum here. I'm not so sure that I wouldn't refer to the whole area around 440/450nm as Indigo rather than violet.