Hey buddy, neat colour scheme you have there. I like the additional claw you added.
If you're wanting to add hight to your model a good way is to give his base some terrain for him to stand on. There are many tutorials online about different ways to do this. Some are complicated but yield really nice results. Basing can also be as simple as gluing some rocks and sand to your base. Either way make sure you have somewhere flat for your marine to stand on (even if they are two separate pieces of rubble.)
Try coating your model with a wash. These are like staining inks that work their way into the typically darker areas of the model. After that, using a fine brush, you can highlight the edges of his armour using a lighter shade of the colour you are painting over.
The highlighting combined with the was is a simple but effective way to add depth to your paint job.
I'd recommend looking up some video tutorials as they can provide you with a step-by-step guide on effective painting methods and will generally give you a better idea of what to do rather than what I can offer
Good luck