Single transistor instead of RGB screen
I'm taking a guess about what you are misunderstanding, but here goes...


  colours are already just different frequency waves.


This is true, but the frequencies involved are very very high. 

\$f = \frac{c}{\lambda}\$.

So for a wavelength of 640 nm (red), you're looking at a frequency of about 470 THz. That's 470 x 1012 Hz. We don't have any technology that can route signals at these frequencies over wires, or emit them using the kind of antennas used for RF.

Instead we rely on the natural resonances of certain materials (the bandgaps of different compound semiconductors or transition energies of phosphors) to generate these frequencies. And different wavelengths require different materials, which is why pixels in a display require 3 separate LEDs or 3 separate phosphors, for example.