I use gimp but I know that for most programs, making a white marking is the same particularly for photomanips so I can try to help you.

First you need to make a selection with a lasso tool in the shape of the marking. Then go to hue, lightness, contrast and saturation. You lighten the area first by moving the bar so that it is lighter. Then you up the contrast just a little bit. After that, you can add depth as needed by taking a selection of a greyish color and going over the areas that would naturally be darker. Hope this helps