Thanks for checking my log out! I wish I had more time lately to work on them.
My suggestion is kind of the same. Look up washes, its a really simple technique and does wonders for you. Pherions suggestions are great as usual too. He is a better painter than I am.
The thing I'm seeing with your guys as well as the falcon earlier is that its like solid color meets solid color. You seem to have the colors down and be able to actually pin point where you want those colors, which is more than I can say for myself haha. So if you took a wash over it all the dark colors in the recesses of the model as shadow would really make the colors you have pop pretty well I think. So that would definitely be my first step.
The way I do it, if you dont want to actually buy washes, is I just really water down some black paint. Then I just lay the model down and brush the watered down paint over the whole model. It kinda looks dirty and all but it darkens everything and the watered paint rolls down in the cracks and where there would be shadow. Then when it drys I flip the model, basically doing this to every side I can using gravity. Then when its all done and dry I take my original color again and paint back over it but don't go over the places the shadow would be. Some people say if you are making your own washes like this to add stuff to them like floor wax and such, but I just used water mostly, recently I added a little soap to the water, it seems to do about the same thing but maybe sinks it into the cracks a little better.
Just something really easy to try if you want, it will probably feel like a lot of work cause you have to basically paint everything like 3 times with the paint, then wash, then paint again but it looks pretty cool when you get it done.
Then if you want to go really crazy is when I'd add highlights. Just small areas around edges and such with a lighter color.
Oh ya, for your yellow parts too, instead of using black paint for the wash I'd probably try and use a brown color(I think Pherion suggested sepia? that'd probably be good but if you dont have it any brown-esque color you like would probably work.)
Another thing, if you don't like those paint jobs on your older minis, don't be afraid to strip them and repaint them. Stripping is another thing that is alot easier to do than alot of people usually think. And then you have a nice clean model to repaint the way you wanted it.
Keep up the work! Hope this helps.