Okay, looked at the Opti Tri and can confirm what you have found, no RGB mode. Menu 8 has 32 different color marcos on it, but no precise control like described in the manual. Maybe Elation has done a revision, and the new software version has RGB mode on it. Only thing I can think of. I would contact Elation support directly and ask them.
Something you can try and do in the mean time to get precise control. Set the fixtures to DMX mode, 3 channel or higher for RGB control, plug in a DMX console, set the desired levels, and unplug the DMX console. The light should hold the last DMX command it received. This means whatever RGB value was mixed on the console, the fixture remembers it. the only issue is, if the fixture loses power or has its address/function changed, it will reset back to nothing or going into whatever standalone mode it was bumped into.