i did a stain colored glass and put a spot behind to see the color being reflected on the floor
and it look nice with blender renderer see pic
i tried with yafray and setting the spot intensity to 10 = Max
and still it is very dark and no color?
what setting is needed to do the same thing with yafray
The answer may be a complicated system of masks and coloured lights.
However Yafaray should help. There isn’t an official release yet but builds are available at yafray.org
EDIT - Actually, you could maybe use HDR lighting with Yafray but I think the effect would be subtle.