Ah, there's the problem, I think.
Firstly,what kind of fixtures are we talking about here? If they are ones with a white colour source and a colour wheel then they will not mix unless you have some on one colour and some on another. If one of your playbacks sets them all to yellow and the other sets them all to green then the colour you have will be whichever playback was raised last.
On the other hand, if the fixtures are RGB LEDs then you need to know how colour mixing works with light.
When using paint the primary colours are red, yellow and blue and green (a mixture of blue and yellow) is a secondary colour.
With light the primary colours are red, green and blue and yellow is a secondary colour made by mixing red and green (the other secondary colours are magenta (red + blue) and cyan (green + blue).
The instructions in the manual tell you how to set up faders that control red, green and blue separately and, using these in various proportions, you should be able to mix over 16 million colours and shades. If you have RGBW, RGBAW or RGBAWUV fixtures and set up faders for the extra colours you could get many more.
(Incidentally the primary colours in painting are actually wrong. Mixing blue and yellow paint only gives green because yellow pigments reflect red and green light but absorb blue, blue pigments only reflect blue light and absorb red so the only colour reflected by a mixture of yellow and blue pigments only reflects green light. Trouble is we have hundreds of years where no-one understood this and, as children, we learn painting before we learn physics and biology.)