Cause of uniform glow in cathode ray tubes
The uniform glow is due to ionization and recombination of the residual gas; it's called a glow discharge. At higher pressure, a spark or arc discharge occurs at much higher current density. Fluorescent lamps and neon lamps operate in the glow-discharge region, and high-pressure xenon lamps use an arc discharge.

Elements of the residual gas can be identified by the color of the glow discharge. Nitrogen and argon in air, for example, glow purplish blue.

As pressure decreases further, density is too low (and path too short) to produce a visible glow discharge, but electrons, AKA "cathode rays", hurled off the electrodes impact the walls of the container, which may glow green in borosilicate glass, and in very low-pressure vacuum tubes, such as the 1G3GT high-voltage rectifier, the electrons impacting the anode produce X-rays (high energy photons), which may also create fluorescence in the glass shell. 

The difference is easy to see in this video, which demonstrates the change in appearance from an arc discharge to glow discharge, and finally to the fluorescence of the glass envelope due to X-rays or electron bombardment. The arc starts at ~20 seconds, the glow discharge at ~26 seconds, striations form ~36 seconds, and by ~46 seconds, the glass envelope glows green from electron bombardment (though it might rather be the green of residual oxygen). This was one of my favorite demonstrations to show students the basis for spectroscopy!