I think you need at least two colors to go with the white and dark green. I think a manila yellow would look good and make the green pop. This site has a good example of multiple color use in trim with non-painted walls: http://theplastermaster.net/outback.aspx
Other color which could work with brick, white and dark green are: lavender, lime, azure, or canary. Are you going to paint the siding as well?
So, another in addition to the beige for the pebbling and siding? I kind of feel like green and beige and white and the red brick will be busy enough. (The only siding on the whole house is that shingle stuff in the side gables, which I will be painting, probably the same colour as I pick for the pebbling on the front.)
Beige and white hardly count as colors. Victorian painted ladies often had multiple colors to highlight various details in the trim. http://www.historichousecolors.com/ItalianateVilla-Schulien.html