Your models are fully painted which is always a plus in my book, and you seem to be doing good on your color variation.
I am by no means an expert but here are some tips that have helped me out in the past. Feel free to pm me if you would like any additional details
Go easy on the nuln oil. Looks like you've applied it pretty heavily to the yellow's which would benefit more from a light agrax or orangish wash.
Add some depth with some line highlights. You could really make this pop with a quick drybrush on areas like the leather straps with a lighter brown to make those edges pop. Same goes for the flat black areas on the weapons but with bolt metal or mythril silver.
Thin your paint / apply in layers. This will also help you out when highlighting the skin and give more overall control on shading.
Look into ink for your yellows. It's a tough color to work with but can look great if you base that area with an eggshell white and go back to lightly highlight with stark white. After that you just tint it with a watered down yellow ink.
if you are going to paint a piece flat black - Try going for a dark grey instead, adding a light wash of nuln oil and going back to drybrush for highlights. It will give a nice dingey look.
Hope this helps!
