Is TeX an option for you?
n=10;
plot(1:n);
labels = cell(n,1);
for i=1:n
labels{i} = sprintf('\\color[rgb]{%f, %f, %f}%s', [rand rand rand], '\lambda');
end
set(gca,'XTick',1:n);
set(gca,'XTickLabel',labels);
Note that this requires rgb values, with each value between 0 and 1.
Also: what is xticklabel_rotate? Whatever it is, the above works for me without it.