Hi Xan,
I've just used three colours on my DH2 tyres, with a 2-toned effect (see Red Baron's post):
for side walls it was Tamiya XF 54 (Dark Sea Grey) with a spot or two of X-17 (Pink)
for tread area it was Tamiya XF-19 (Sky Grey) tinted with X-17.
Looks OK to me but there must be lots of variations. Tyres often have a patchy look superimposed on the often two-tone appearance. All very subtle.
Cheers,
GrahamB