Originally, the red and white case options were ordered from Motorola when you purchase the pager. The older M03 monitors were available in black and red, and Motorola just expanded the color offerings to appeal to fire and ems as they transitioned from desktop alert monitors to tone and voice monitoring pagers. The reason for the colors was to appeal directly to the fire and ems markets. The the I's and the II's were available in dark gray, red, and white.
I remember buying 35 of the white Minitor I's for the ambulance squad I belonged to in the '70's and '80's. As the I's aged out, we replaced them with white Mintor II's. If I recall correctly, the color choice was a no charge option as part of the pager purchase, at least with the II's.
I don't think anyone has any sales figures on the different color cases for the II's. But I can tell you that they were darned popular. I still have a small pile of VHF II's in a box somewhere. They're not in particularly good shape. I will look when I go over to the ambulance building in the morning and see if any of them are good enough to fill out your collection.