Dave Williams of Kalamazoo, MI states his perspective on color.
I have read all the posted articles, found them interesting, but find them inconclusive – yet, there are yet many unanswered questions. I am certainly no expert, but may I put in my two bits worth?

Back in Carder’s years of glass making – purity: how was a new batch of glass made? Were the ingredients precisely
measured and put into a clean crucible before melting and mixing? There must have been some variation, and the materials possibly did not always come from the same source.

Also, for articles made with layers of different glasses, there must have been some influence by the individual
gaffers experience and desires of color.

I wish we could call upon the past glass chemists and ask them questions. Paul Gardner knew some of them, and knew
something of glass making himself.

Then James Jory of Albuquerque provides a picture of four Loetz vases labeled the Hearts group that should have identical colors, and reaches a similar conclusion to Dave Williams. He says:
I have been following the discussion about glass colors with a lot of interest. In my possession is a group of four Loetz vases with identical decorations (http://www.flickr.com/photos/cardersteubenclub/?saved=1) that demonstrate what happens if one of the colorants happens to be quite volatile. The excess application of heat, either by degree and/or duration, caused the evaporation (or perhaps the degradation) of the blue colorant to different extent as seen in the attached photo. Evidence that one named color may have several different shades.

Additionally, one can see in those pieces in which the blue has faded there is a reddish blush, correlated to the same degree, caused by the salmon pink base glass bleeding into and mixing with the coppery gold surface layer. Another possible reason to be careful when attempting to identify a particular color.