Timbre has Synesthesia!
Well, sort of. The latest version of Timbre now displays the timbre of your instrument as a colour. The colour is chosen based on the relative strengths of the first few harmonics. I played around with several ways to map the harmonic information to colour and Timbre is using the best one I've found so far.
The general intent is to get a "pure" flute like sound to be Blue, a clarinet type sound to be green, and brassy sounds to have more red. The actual implementation is more complicated and involves delving into the world of colour spaces.
In practise there is significant overlap between the colours of instruments. A serendipitous feature of the way the colours work, is that there is significant variation in colour with tone of a single instrument. It's fascinating to experiment and see how timbre can be changed by the musician.
A quick note about synesthesia. In all the interviews I've heard with people that have synesthesia, the mapping between sound and colour seems to be closely related to the note that is heard. Timbre works differently. The mapping is based, not surprisingly, on timbre.