At present, all transitions and effects default to using BLACK for backgrounds where the resulting effect or transition uses less than the whole screen. While this works, it might be handy to be able to set what colour is used for each effect or transition (white, pink, whatever). Practically this would be a button like the one for chromakey on overlays - the user can manually select a colour, or a double click allows them to use an eye dropper to select a colour from the underlying clip.
In the meantime (and I was asked about this on youtube after doing a couple of tutorials for AVS Video Editor) the workaround for this is:
1. Create the transition/effect as usual;
2. Produce as an AVI/whatever and trim resulting video (using external editor like VideoDub) so that only that transition/effect remains;
3. Import video; and then use as an overlay. Set the chromakey to eliminate the black (tough luck if there's black in the clip);
4. On the main timeline insert the colour wanted.