It's okay to experiment a little, and have fun with your work. If you do want to experiment, consider the properties of the fiber you wish to use, and act accordingly. For example, cotton embroidery floss can have lovely results when used on knit fabrics, but has little give to it, so use them in areas of the garment that will not be stretched. Also choose your embroidery needle carefully. Generally darning or tapestry needles with blunt, rather than pointed edges work best.
Sharper edges tend to catch and split yarn, while blunter ends glide through stitches more easily.
If you have never done duplicate stitch before, practice before you begin work on the actual garment. Experiment on a knit fabric with larger stitches. This technique can be tricky at first, so don't get frustrated. Your goal is to keep stitches loose enough so as not to pull the fabric, while at the same keeping them uniform to create an even look. When you are doing the actual color work, follow the same direction as you would for knitting.