I don’t think that white – in GREEN circle would work well. Too similar to the existing white + in a green circle.
But I do take your point about the red colour. So perhaps a rethink of both the expand and contract icons is called for.
All the icons in that theme are taken from a single independent design site, so I would prefer to stick with those rather than getting into designing something new. That also allows us to read any of them from a single image file, icon_sprite.png. But it is a large set so there should be alternatives. Take a look yourself and see if you see anything suitable either on that image file, or here: http://www.famfamfam.com/lab/icons/silk/previews/index_abc.png