Vignetting as opposed to hotspotting, and the color shift problem--my uneducated analysis.
I did some quick analysis with my setup (I said above I had a 60"x80" screen--WRONG--I have a 72"x96" Dalite Highpower screen. Anyway the front of the projector is about 11.5 feet from the screen, and I am sitting about 18 feet from the screen (I have a Barco 800). Now if you analyze how light hits the screen, there is a cone of light from the green head on, and offset cones from the red and blue. The offset cones can cause red or blue shift because the light reflects differently out of their cones than the green. The red and blue are reflecting at slightly different horizontal angles than the green. Thus considering that a high gain screen of 2.5 has an reflectance angle of 25 degrees, some red and blue is concentrated outside that cone on either side, so as you move out of the center cone you might see some red or blue on either side of the screen. This is purely a phenom of brightness of the reflections of the 3 crts, I think. The image coming off of each crt must be dead center so that it comes out dead center of the crt. If it is off slightly, it will be refracted unevenly through the lens which could cause some color shift.
You should be able to draw this on paper. Draw 3 different cones coming out and hitting the screen and reflect those at the same angle they hit the screen--notice that red and blue are outside the 25 degree cone on either side. In a sense this causes the vignetting as well. It is also related to the fact that the center of the lens sends more light to the screen than the edges of the lens, and this affect gets magnefied on a high gain screen. A curved screen overcomes some of these problems by reflecting light in a more even pattern from each crt, while further reducing the size of the cone of light reflected back.
There is a sweet spot that you will find that does away with most problems. Mine is at 16 feet if the projector is about 12 feet. For those of you sitting closer to the screen than your projectors (especially you ceiling mounts)--step back a bit and see if this doesn't improve the picture.
Here is also a secret that I've used when tubes get bad--but you can use it to spruce up your pictures by 20-30% in increased sharpness. First get a high gain screen that matches the low gain screen you have in size, but do not turn down the brightness or contrast. Then cut three rings out of black paper the size of the lens diameter. Then cut a hole in the paper at 90% of the diameter of the lens. Tape these rings lightly over each lens and wholla--be prepared for a film like picture the likes of which are beyond your wildest dreams--so I exaggerate--but hey it will look better believe me! If brightness or contrast is not a problem, try reducing the opening to 80% etc. I have used this trick on blooming tubes (you'll have to recalibrate your whites of course to make up for the reduced light of one tube), and it is a quick fix to regain some sharpness. Downside of this might be some more vignetting--but hey it costs almost nothing to try if you already have a high gain screen--in fact I'm going to do this tonight with my new screen and see how it works (I did this about three years ago with an old projector and it blew me away). Of course anyone with a little photography knowledge knows that I'm simply increasing the f/stop of the lens which in turn increases the lens focus range and therefore sharpness--especially in the corners.
Also for ceiling mounted screens. Note the angle at which light comes out of the projector and hits the center of the screen on your wall. (Pull a string from the crt to the center of the wall and measure the angle between it and a stick sticking perpendicular out from the wall. Then (with the help of someone), put the string on the wall and pull it out at the same opposing angle to where you sit. If it is over your head or below your head, you are not getting the full advantage of the screen. Now get this--you simply need to bring the bottom or the top of the screen out and reflect at a slightly different angle (some keystone adjustment may be necessary--and this works best with perm mounts, although wall hangers can be adjusted if you are willing to mount them away from the wall), and boom--you've got light reflected where it needs to--in your eyes. For floor mounts, since the light reflects directly back to the projector there is little you can do but mount the projector closer to eye level--best is to have it so that when you are looking at the image, you can see the bottom just over the top of the lens--looking from the back of the projector. It helps to move the images on the tubes down so that the images projected are close to the ceiling--this brings your screen up and causes the reflected image to be at a better retroreflective angle.
Disclaimer: beyond the above tricks, my science falls in the realm of hard knox (pun intended), not mathematics.
Bob--let the sputtering begin. "But, but, but----now you're a motor boat".