xypplot (lattice): colours of lines

Using data from a multicenter study with a parallel-group design comparing
two treatments, I plotted each subject's time change of X after stratifying
for center:

xyplot(X ~ time | center, type="l", panel=panel.superpose, groups=subject)

Now I want these lines to take different colours according to the variable
"treatment". Any help?
Umberto

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