I am almost sure this has been asked before, but I can only find answers for NTSC: Is the IIgs capable of outputting a PAL color signal on the composite port?
I can't get it to display color on neither a LCD TV nor on my Commodore 1084S, that works perfectly for RGB. NTSC mode is in color, however.
RGB is fine for GS/OS and text, but most games look much nicer on composite. For example Prince of Persia looks much nicer on my IIe than on the IIgs.