ggplot: adding layer using different data, groups and also controlling appearance

I have a more complicated function I am trying to write, but I run in to a problem when I want to
add something to the plot from more than one data set while simultaneously controlling the
appearance of the additional layer.

# Toy data:

foo <- data.frame ( x = 1:4, y = 4:1 , membership = c( "A", "A", "B", "B" ) )
bar <- data.frame ( x = 1:4 + 1 , y = 4:1 + 1, membership = rep ( "C", 4 ) )

# This works, and produces a sensible (unified!) legend:

foo.gg <- ggplot ( mapping =  aes ( x = x, y = y , colour = membership ) )
foo.gg <- foo.gg + geom_point(data = foo)
foobar.gg <- foo.gg + geom_point ( data = bar ) # works fine, creates a unified scale showing "membership"
foobar.gg

# This does not work:
booboo.gg <- foo.gg + geom_point ( data = bar, colour = "black" )  # so far, so good, but...
booboo.gg # it will not let me over-ride the colour specifications when it actually tries to construct the graph

The error message is:
"Error in `[.data.frame`(df, , var) : undefined columns selected"

System Information:
R version 2.7.2 (2008-08-25)
i386-pc-mingw32
ggplot2_0.6

My goal is to be able to add the additional layer and control its appearance separately from the appearance of
the first layer. Possible?