When working stranded colourwork you will need to work with at least two colours at a time. Using both of your hands to hold the yarn allows you to ensure your yarns never get crossed. In addition to this it will help keep the dominant and background colours consistent.
The yarn you hold in your left hand (as it come from below) will be the dominant colour. This is usually the main pattern stitch. The background colour will then be in your right hand. You can opt to swap these around but once you keep it consistent it works well. (I'd suggest even swatching to check which you prefer).
