Good attitude bentoad, keep your paint thin and your pressure low, and build up the layers, that'll help you with tone and depth to get that 3D look, when painting on black a lot of your shading can be done by allowing the base colour to show through, and adding more paint in some areas than others will vary the tone. Keep at it, it'll click, and keep plugging away at mitches excercise sheets, they really will pay off in the long run. I know, because not having a clue when I started, I went my own merry way, and when I eventually did some research, had to unlearn a lot of stuff.