By looking at your digital work, I instantly see issues with Contrast, value, lighting, temperature, perspective- the list goes on. The tats are way cool, looking at wolverine gave me this idea that you have great control over value but you need to just practice and practice, do values scales constantly, and don't worry about color yet. Just focus on the basics, Anatomy, Perspective, composition- and draw from life
A GREAT strategy I learned. Grab all your paint stuff walk outside of your house and walk totally straight for 5 minutes exactly. Then sit down RIGHT THERE, and find the nearest intrerestig thing to paint. Do that once a nice, and you should see improvement in no time!!