Here was the exercise for working your yellow colors beyond the "Yellow Space Marine" effect.
Just like the previous color theory videos, this one describes how to use those warmer and cooler versions of a color to create contrast and interest.
I made the feathers the lightest and most 'pure' colors, with the amber gem on the end of the staff, with the idea of making that a focal point.
Many of your 'shadow' colors are brownish, but making these redder or greener allows you to get a whole range of colors. I also used some leaves to do more orange yellow.
This was very fun, since I have only been using yellows as part of NMM or painting Imperial Fists. The one color set I left out was the fluorescent paints, which is a key component of the Gold NMM.