We are contemplating doing "City of Angels" and I would like to be able to avoid the half color/ half black-and-white set design.  My though was to use U/V light and make the change with lighting.

Try using 'Sepia' gel instead say as a wash from floods or even on the cyc?

{With blacklight fluro tubes & UV Cannons, the effect is made by the use of 'Woods Glass' [which is coating on the glass envelope, developed by William Woods]} Something else you may consider is the 'Samoiloff Effect', which is not done that much these days.This works well with black & white fabrics & paint pigments. It was used in the 'Ascot Scene' of My Fair Lady. Where they were all dressed in black & white costumes, the black turns to green on cue. {This was first developed by Adrian Samoiloff & performed at the Hippodrome in London back in 1922. By the use of Red & Green colour saturation, which he perfected when Strand Lighting introduced their box trough batten lights.}

I don't think for one moment, especially these days, there is any issue using any thing else beyond blacklight UVA fittings. Unlike back in 50's which used discharge UV lamps, which were close to the shorter xray wavelength.

I remember seeing this on Broadway, and was totally blown away by the effect. They had a colorful set, (dominated by the color yellow, I seem to recall), and in the next scene, the color was totally washed out. Everything was suddenly a believable monochrome!