I guess you're young, because the whole selling point of those colored phosphors was ergonomics. However I will grant it's not the *color* per se that was the selling point; the green and amber phosphors used in those tubes "decays" more slowly than the standard P4 white phosphor used in TV-grade monochrome tubes, IE, it emits the energy absorbed from the electron beam more slowly. With a standard white tube a given pixel is very bright at the moment the beam is hitting it but fades almost to black before the beam hits it again, so the display essentially strobes at 50-60hz. The green phosphor, on the other hand, glows less brightly while absorbing the beam but takes a decent fraction of a second to release the energy so the display has a steadier glow, which, as noted, was claimed to reduce eyestrain.
(Personally I can vouch for it, mostly when talking about 12" or larger monitors running a the 50hz MDA/"Hercules" refresh rate; white MDA monitors will burn your eyes right out of your skull. With 9" monitors the flicker from P4 phosphor is less noticeable because the human eye is more sensitive to flicker in the edges of the visual field rather than the center.)
The downside of P1 green phosphor is it actually takes so long to decay that moving objects on the screen leave "ghosts" behind them, which is a bigger problem with graphics/GUI vs. text displays, and the phosphor is also more prone to burn-in.