How to configure Windows RDC to use 16-color mode?

I have found the answer myself, so sharing it for further reference of whoever may need it.

The solution is to open the .rdp file (%USERPROFILE%\Documents\Default.rdp by default) and edit (or add if missing) the following line:

session bpp:i:8

The number (8 in this case) sets the color resolution in bits per pixel. 8 (for 256 colors) is the lowest value available through manual editing (while the lowest value available through the RDC GUI is 15).

Other .rdp file commands can be found here.

PS: I am also glad to mention that my practical problem (unintentional but unavoidable video streaming jamming the whole RDP channel) is solved completely! The thing works lightning-fast now. So the idea to lower the color depth was right what I needed.