Is it better just on white?
Again, on glass block with white paper under it, overhead (ceiling) flourescent tubes.
I was going through this book on photoshop, and I came across something that covers your problem; about the background looking grey-ish when put againt a bright subject. I'm sure you must've considered this, but in any case, here it is-
1) The above image required special attention to ensure the
background went to pure white. Yet I did not want to blow out the
highlights on the white shirt or on the models blonde hair.
2) A Levels adjustment was applied to make the highlights go as
light as possible on the shirt, but without losing any detail. But
the background still needed to go whiter. I made a quick selection
of the background area, feathered the selection by 24 pixels and
fi lled this with white.
3) There needs to be a gentle tonal transition between the white
cut-out and the edges of the subject. If not, the edge detail will
become lost in print. I deselected the selection and set the history
source against the original history state (see the circled history state
icon in the History palette) and carefully painted around the subject
edges, especially around the fi ne strands of hair, to restore detail.
4) If we check the fi nal histogram you can see a slight gap
between where the highlight tones of the subject tail off and the
white background cuts in.
Hopefully this helps.