That’s the comical thing. In my original manual and the copy found online, Tandy says this is a 16 color RGB monitor:
https://manuals.plus/m/592b4454659e114409ba085914c51a7bd0f03b1158a49d7c84521bf1bd10611f
However, I recall in Radio Shack catalog listings and ads from way back in the day, and in present websites nowadays, they often refer to this as a CGA monitor(likely because they are of the Amiga breed and this term seems to be used loosely in that segment of retro PC society), whereas on the PC scene, CGA refers to 4 colors or composite out to a TV that produced more colors through artifacting:
https://www.reddit.com/r/crtgaming/comments/a5ekuv/weird_aqua_color_and_lined_image_on_monitor_tandy/
Basically, the monitor connector has 9 pins which was common among CGA monitors of the time, unlike later VGA which used 15 pins. The Tandy 16 color standard straddled EGA and CGA, and competed originally with the PC Jr. upon release, which also had 16 color capability, which was remarkable way back in 1984 when every other PC could only display 4 colors. The Tandy 16 color mode is incompatible with EGA, and special drivers had to be written to output properly.