I think this is tricky.
We all have an idea of what "true red" is, and when things are shades of pink or marroon we know we can say that a thing is "less red" than the thing that is fire-engine red or brick-red, or that her lipstick is the reddest of the three girls. I think the same is true for yellow.
But with other colors, like blue, this can be hard. Is navy blue "true" blue and a sky blue "not as blue"? I don't think so. Is hunter green more green than kelly green? Or less so? I don't know.
I have never study graphic arts, so I can't talk about colors very well. I just wanted to say there isn't a simple answer that will always work.
However, I completely agree that you can say "yellowish" and other things of that nature.