UHP lamps tend to be red deficient, so if you push up the individual RGB contrasts, you will find that green and blue are higher than red. Normaly you would reduce the green and blue to match the red, but in doing so reduce your overall contrast, and the contrast ratio with respect to black. You may notice the overall brightness of white is lower. You haven't changed black, but now your CR is lower.
If you use a filter instead of the projectors digital RGB contrast adjustment to correct the color, you keep the RGB contrasts high, and although overall you have a dimmer image, the diference from black to white is higher because you never lost any contrast by reducing them from within the projector.
An FL-Day filter reduces mostly green, and a yellow filter reduces blue for example. Finding the right filter for the job can be tricky.
It can be a bit trial and error, but Colorfacts (and the other two as well I would say) will be able to tell you how out of balance the RGBs are, and by using a filter, how much you have corrected them - with CF you have RGB bars that show you how high each color is in relation to the others, so by just placing the filter in front you will almost instantly see the effect. You will always have to adjust the RGB contrast to some degree because you won't find a perfect filter, but gaining 300:1 CR with a filter is possible.