Originally Posted by sspears
You want to set color and tint using only the blue filter. A blue filter is not 100% accurate. The only 100% method is for the display to offer a true blue only mode.
Don may chime in and explain why green filters may never work.
It's true that the only sure way is to have a blue-only mode of some kind. But in fact you should be able to use a green-only or red-only mode and do the calibration and get the same results.
The whole point of the color bars is that the bars represent every combination of red, green, and blue at a single level (generally 180). So the white should end up as RGB (180,180,180) and cyan should be RGB (0,180,180), and red is RGB (180,0,0), and so forth.
So if you view through a blue filter, you are just verifying that all of the bars have the same level in the blue channel. If you view through a red filter, you'd be doing the same for the red channel.
Assuming that the display uses the standard conversion matrices for converting between YCbCr and RGB (which is not always a good assumption), then when color and hue are set right, all three channels should be at equal levels. If color or hue isn't set right, all three channels will be wrong. It's not possible to make the blue channel right and the red or green channel wrong, or red right and blue and green wrong, etc.
So blue got chosen to be the traditional channel to check color and hue settings. It could have been red or green. But you only need one, so BVMs have a simple mode switch that puts the monitor in blue-only mode.
Filters would be just as good as a blue-only (or red-only or green-only) mode if they perfectly passed one color channel and removed all of the other two. But in practice, no practical filter does that perfectly. There's always a little red and/or green that leaks through the blue filter. If it's a small enough amount, it has a negligible effect on the calibration. But if it's large enough that you can see it, there's no point in even proceeding with the calibration because it won't be right.
When you look through the blue filter, look at the red, yellow, and green bars. All three of them should turn black. If they show even a hint of red, yellow, or green, or show different brightness levels, the filter is passing some green and red, and you shouldn't bother calibrating unless the settings are waaaaay off and you just want to get them closer.
If you have all three filters, whichever one of them makes the black bars the blackest (and the most alike in brightness) is the one to use. The calibration procedure is the same no matter which filter you use.
Once you've calibrated with one filter, you can use the other filters to check that the color decoder is using the standard matrix, but again, since filters are notoriously unreliable, I wouldn't trust the results. If your projector allows you to turn off colors selectively, then those results can be trusted.
As Stacey mentions, the green filter is near-impossible to get right, because the color primaries of any practical display have overlapping spectra, and there is often a tiny little notch of frequencies that are only covered by the green primary and not the red and blue. The red and blue primaries only overlap one other primary each (green, in both cases), and thus have a larger range of frequencies that don't overlap.