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?

Umm, you should already have lines colored differently by different 
'subject's. Do you want to further differentiate by treatment ? 

If that's so, there's probably no good way. You could try creating a new 
factor combining subject and treatment and use that as the groups argument, 
e.g.,

factor(paste(as.character(subject), as.character(treatment), sep = "/"))

Deepayan