The more you practice, the better you'll get at judging color saturation by feel. When I say "by feel," I mean that when you're stretching the skin during a tattoo, pay attention to how it vibrates under your needles. Comparing that sensation to the visual results will help train your brain so you can instinctively tell when you're packing in enough color.
Don't be afraid to return to your liner at the end of a session. This will allow you to finish off your work with the attention to detail your client will appreciate and leave the tattoo looking clean, and your colors feeling confidant. Check for any gaps in consistency where your color is approaching a line and look for any imperfections. When you look back and notice how those little modifications make the piece really pop, you'll be amazed!