I dont think primary colours and the theory of, really come into it.
Grass is a lot more yellow in colour than green.
The theories above would indicate that changing the blue slider would effect the colour of the grass too. Which is not the case.
Your also dealing with RGB, so green is a light in its own right.
yellow is actually the combination of red and green in RGB, which is very different from saying green is the combination of blue and yellow.