To use, select the color of the shadow you're planning on blurring and use the colorful blur tool. This tool only works on a layer with only shadow, if you try to use it on a merged image it will not give the desired effect.
You can change the settings to give different effects. You can turn on "Change Brush Tip Color" to give a SSS effect, and "Change Brush Color by Stroke" to give a randomized color effect. Turn both of these off if you want to choose your own colors. Change the hue and saturation as you wish.
You can also use this brush as an easy way to get an SSS effect by turning off the color mixing and using it as a regular brush. Since the colorful effect is based on the color of your brush, you can either select the shadow color or a color you want to "hide" within the drawing.
Normal blur


With Colorful Blur

When used as both a regular brush and blur when choosing own colors by turning off color jitter as stated above.