The exact same thing happened to me when white balancing just before a shoot last Sunday. I had my back to a bunch of floor-to-ceiling windows in a restaurant. Inside the restaurant was incandescent lighting that was active but quite dim. Outside was mixed clouds and sun. I used a pure white foam board to balance on and when I did the image in the EX1's LCD screen turned a bright green. And I mean bright. Never seen anything like that before on any camera.

- Don

Now I am using flur lights 5600k... No mixed light at all. My walls are a off white / tan color.

I manually white balance with an off white paper or 18% grey card and i still get this
unwanted green tint.

The green tint is not that bad, but very unwanted.

The skin tones and everything in the shot looks great except the off white walls.

Weird

The amount of green in "professional" 5600 K fluo lights varies from brand to brand. Taking out all of the mercury (the source of the green spike) cuts the output too much, so manufacturers try to balance with other elements, with varying degrees of success.

Printer/copier paper has gotten "whiter" over the last several years. Some of these extra "white" papers photograph magenta under correct 3200 K tungsten, but look very white to the eye. Setting white balance on these magenta papers will give a slight green cast.

Some days it seems almost everything is harder than it looks. Buy a DSC labs white card.

Seconding what others have said here about using a DSC white card, or if you can't, at least find a good substitute and keep it with you at all times - and use that same card consistently. Balancing on whatever I can find lying around (t-shirts? eegads) that looks white to my eyes has gotten me in trouble in the past, and I now always have my "real" cards in with my camera wherever it goes.

I'm not near my gear at the moment, but isn't there a matrix setting specifically for fluorescent lighting? Has anyone here found that matrix setting useful?