You're right that there needs to be more transition from dark to light, but I think you're still missing a lot of what's conveyed in carlobarley's example. The main thing that jumps out at me is your colors. You chose very saturated, almost crayon-like colors for your shield. In the reference the blue is almost gray, and…
Just like JadeEyePanda said, you actually need to learn how to paint before trying handpainting, you clearly lack the base, you don't know about colors and lights theory, for example a shadow is never really black. A color isn't only made by a gradiant of the same slider, you need different value etc. Try painting that…