When you shoot RAW, settings which impact color are not applied by the camera (white balance settings are ignored for RAW images.)
This means a very real color cast would not be removed by the camera and must be removed by your computer (e.g. using Lightroom).
To correct white balance when shooting RAW, take a photo a "neutral" color calibration target. A "gray card" is the common target of choice, although there are many other options. The particular tone of "gray" is formulated to be color-neutral. Any level of RGB in which the "red", "green", and "blue" pixel values are identical will result in a "gray" pixel (something on the continuum from black to white.)
e.g. in an 8-bit color system (values of red, green, and blue can range from 0 to 255), an RGB color value of 127, 127, 127 (for red, green, and blue respecitively) would be a "middle gray" color. But if it were 127, 140, 127, then you'd have a gray that had a slight "green" cast to it.
To eliminate the color imbalance, you pick the white balance option in Lightroom, use the dropper to select a pixel in your image, and then click any pixel on "gray card" target in your image. The software, knowing the RGB values should all be identical, will detect which colors are out of balance (and how much) and will correct for the entire image based on that sample. You can then copy the color correction to all subsequent images (as long as they were taken in the same light.)
If you mouse over the gray card, but you software reports that the RGB values are all identical (or just extremely close) then this would indicate that the camera is actually correct... and it's your monitor adjustment which is causing the image to render images with a greenish tint.
Tim Campbell
5D III, 5D IV, 60Da
