Blending modes can be accessed from any of Photshops painting tools or from the layers themselves.
Screen mode takes the base color and adds light to it.
Basically it's an easy way to get a lighter value of your base color. Great for adding hightlites. With that same base color selected. It increases the light value of the fill color by screening it against itself until it reaches White. You can also screen against the base color with a different color for a varied effect.