Adult coloring is fun, relaxing and stress relieving. I believe that art is about expression and not a competition. That being said, a few tips, tricks and techniques to elevate your work wouldn't hurt so here goes :)
1) Create a color chart out of your markers. You really want to make sure you know what colors you are coloring with. Take the time to scribble with each marker on a separate sheet of paper so that you know for sure the shade of each of your markers pens or colored pencils this way when you start coloring you'll be sure you're using the shade you wanted and you won't have to find out too late this wasn't the color you wanted.
2) Put a paper underneath the page you are coloring. If you are coloring in a coloring book (as opposed to a isolated coloring page) put a card stock paper underneath the page you are coloring so the ink won't bleed onto the next page and ruin it for you.
3) Let your imagination and creativity run wild with the colors. Coloring is about fun, happiness, creativity and expression. Don't necessarily worry about trying to make the colors or images realistic, just do what is fun and pretty to you.
4) Consider repeating color patterns on a coloring page. It gives the coloring a hypnotic, pleasant reassuring effect.
5) Experiment with contrasting colors. Contrasting colors work well to emphasize the colors you are working with. Use neon markers with regular markers and black markers. Use vibrant colors together with pastel colors. Use light markers together with dark markers. Use metallic markers together with regular markers. The contrast will make the colors on the page pop and come to life.