It very well could be your roots exposed, giving the yellow appearance. However, there is a proper way to brush:
Point the bristles of your toothbrush toward the gum line at a 45 degree angle. Gently push the bristles under the gum. Then, over 2 teeth at a time, move the brush in a vibration like manner. Do this for about 10 strokes and then move on to the next teeth.
And make sure not to brush too hard, as this can cause gum recession, resulting in exposed roots.
Improper brushing can leave plaque behind, which also leaves a yellowish tint. Hope this helps!