[rab] Even if colour were a continuous 3D space, any finite collection of CD spines could still be sorted by colour because the space of CD colours wouldn't be continuous (indeed, because CDs themselves are discrete items, it seems to me that even an infinite collection of CD spines, coloured from a colour space of arbitrary dimension, would still be sortable by colour, but I must admit my grasp of such things is not what it used to be, if it ever was).
Yes, if you imagine a map of 3D colourspace, your CD spines are simply a series of points in that colourspace with close neighbours joined by a line of your devising. So your shelf represents a one-dimensional journey (as it were) in that 3D colourspace. If your CDs were reasonably well spaced out, then there are many many possible ways of doing this. I only know this because of my similar journeys through the three RGB dimensions when creating Acre Street moves. :)
[matt, Projoy] Well yes, I suppose you could order colours by their hex rgb triplets, or Pantone number, or whatever. But that's not the way I chose to do it.
[Projoy] Pedantically, RGB hex colours for Acre Street moves aren't taken from a continuous space to start with, so it isn't quite the same problem, but on the other hand, it is. As you say there are any number of ways of connecting the dots and thus of sorting. Some of these may correspond to different ways of expressing colour in N dimensions (HSV, YUV, Lab, CMYK, etc), others may be more arbitrary. In the case of CD spines, the issue is more complex because they may be printed in multiple colours, and may include special inks or other production effects that don't fit into any such colour space (metallic, dayglo, holograms, fun fur, etc), which would have to be incorporated into any sorting scheme in some (probably arbitrary) way. [rab] So don't be coy -- how did you do it?