I would study up some color theory books really learn to push your color pallet so you can push colors fwd and back with ease. I feel this should almost always be exaggerated in a tattoo because your looking through the skin to see the image. If your blacks that were further away were more of grayish blue it would make your fwd blacks look closer. Also some edge work in the tattoo would help as well studying how certain edges react in certain light, rolling edges vanishing edges etc. All this can be done pretty easily with one trip to barns a noble and a few hours reading.