Overall I'd say your colors are not dark enough for some of the references. And you are picking several values for each element which is fine, but I wouldnt do more than four. They will all have a purpose in the value structure of illustration. Use them for their purpose place them how they relate to each other in the reference. Don't forget you can use glazing techniques at the end when you are making a strong value structure.
I recommend trying to learn to mix color. There are only Three properties of color.
Value (light or dark)
Saturation (dull or bright)
Hue (warmer or cooler) (compared to what it is next too, or when you are mixing from ref)
Every time you pick a color you have to decide each of these three things over and over. If you want to learn to paint, and mix your own colors, that is the formula. "Okay, is this lighter, or darker? Is this duller or more saturated? Is this warmer or cooler?" All in relation to the reference. If you are doing this digitally it is simple and quick with the color box of whatever program you are using.
Also break up your colors into groups regarding the value structure of light and dark. Or "Notan". The light group will have no more than a couple values for midtones and highlights, with some hue variation for skin tone usually, in the mid tones. The darks will have the same thing, usually the terminator shadow and the bounce lights hitting the shadow side. The only law you have to follow as an artist to render form is the separation of the light and dark. Howard Pyle wouldn't let his students advance until they understood this. The edges inbetween each one and getting it right is called modeling/rendering. Once you start observing those nuances you will find your paintings going much faster.
