What colour's that? And when?

The Welsh traditionally had no words that directly correspond to the English words green, blue, grey or brown. The word "glas" matched part of what English would call green, but a totally different word covered what (to English) would be another shade of green. That term also covered blue and some of grey. I knew that certain cultures made different boundaries between colours, but Welsh was a surprise - the physical experiences and their significances must be very similar on either side of Offa's Dyke.

The word "grue" to a logician/philosopher refers to things that are green before the year 2000 and blue afterwards. Similarly, "bleen" things are blue before 2000 then green. It was introduced in 1955 by Nelson Goodman as a way to discuss potential paradoxes in predicate logic. The choice of the year "2000" in 1955 and subsequent writings was of course shorthand for "some point in the future" but now, in 2007, is better understood as "some point in the past." Thus one inherent paradox of predictate logic no longer applies, and we can safely use it.