Spectralism
More on the Spectralist tip... [and another big Xpost, sorry]
Yeah, Ligeti not one of the Spectralists, although he was probably an influence in some way (he's been an influence on most composers of the last 40 years in some way or another). Similarly Xenakis I suppose. If you like either of these, you might like spectral music, it can have that colouristic wash effect, as well as quite brutal moments. Spectralism itself was a movement of mainly French composers that started in the 70s, off the back of IRCAM (the sound research centre underneath the Pompidou Centre). Michael's explanation of the technical aspects is a good summary, but as well as being musically linked, the composers also shared some radical political affiliations too (which don't get much talked about); overall, think an extension of Paris '68 into the 70s.
The big men from France are Tristan Murail, Gerard Grisey - who died suddenly in 1999 - and Hugues Dufourt, who came up with the term. There's also a German-based strand of spectralism; many of the composers here studied with Stockhausen, although he's definitely not a spectral composer himself. Peter Eotvos, Claude Vivier and Mesias Maiguashca are three names there - Eotvos I can easily recommend. And there's a Romanian strand too, oddly enough: look out for Iancu Dumitrescu (who seems to appear on Resonance a lot) and Horatiu Radelescu among others. There's also a sort of second-generation spectralism, where the music of, in particular Murail and Grisey (both of whom are great, IMHO) has influenced a number of other composers - see Kaija Saariaho, Magnus Lindberg, Jonathan Harvey, James Dillon and Eric Tanguy.