[R] coloring individual points in lattice xyplot

[Gabor Grothendieck]

If I wish to color groups in xyplot I can do this:

   library(lattice)
   x <- 1:10
   y <- cbind(x, x+1)
   xyplot(y ~ rep(x,2), group = col(y), col = 1:2)

How do I color different points differently within a group.

For example, I want to produce this plot (except that I only
want to have two groups, not 11):

   xyplot(y ~ rep(x,2), group = c(rep(1, 10), 2:11), col = 1:11)

I am thinking of something like this (although
this does not work, its just to get the idea across):

   xyplot(y ~ rep(x,2), group = col(y), col = list(1, 2:11))

where, in general, I have a list with one component per group
whose elements are scalars to color the whole group or
vectors one color per point in the group.  I don't know
ahead of time what the list is.

I am looking for a general approach to this within the lattice
xyplot plot framework; the above is just an example.