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About 26 years ago I was working at some defense contractor, and we were using a few red and green color CRTs. One electron gun, you vary the very high voltage to change the color. This changed the energy the electrons in the beam would have when they hit the special phosper mix on the tube face, and thus cause the different colors from red to green to glow. No blue. At first you'd think there is no convergence issues as there is only one electron gun, but the very high voltage varying causes the image size to bloom or shrink. So we had to modulate the amplitudes of the horizontal and vertical deflection feeding the yoke. What fun...
This system was used only for simple graphics (like heads up displays in fighter jets) and not for displaying live video images. Disconnect the blue gun feed on a normal RGB CRT and you can get an idea what this above would look like. SO this would not be a tube you could sell to consumer TV use. Looks bad without blue. And you trade the convergence circuits used for shadowmask 3 gun CRTs for the deflection modulation circuits. You don't come out ahead in cost, and the picture looks awful, so there's no point in building color TVs this way.
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