[R] background color in strip.custom()

[Sven Garbade]

Hi all,
how can I change the background color in lattice strips according to a
factor level, eg:

library(lattice)
x <- rnorm(100)
y <- sqrt(x)
f <- gl(2, 50, c("A", "B"))
xyplot(y ~ x | f)

I like to change the background color of the strips according to the
levels in f and tried several things like this with no success:

xyplot(y ~ x | f, strip=strip.custom(bg=c("red", "green")))

Is this possible?

Thanks, Sven