While not an area I've had experience with, I decided to explore your question in Photoshop 7.0. Upon selecting a brush and opening the brush palette, I saw that one of the brush dynamics options is "color dynamics". When enabled, that provides options to control various "jitter" values from 0-100% for hue, saturation, brightness, and foreground/background, plus two additional options for purity and control (none, pen pressure, pen tilt, fade, and stylus wheel). With a white background for the workspace, I set a foreground color of green, a background of red, and selected a brush pointer style similar to a hair strand. I then enabled color dynamics and set the HSB and fore/background jitters to 100%. Not knowing what to expect, I thought I'd see two colors alternating when, in actuality, I saw multiple colors as shown here: http://jazzdiver.com/images/color_dynamics.jpg
I don't know exactly what is happening, but this little exerise seems to suggest that the brush dynamics are quite powerful. Perhaps they could do the same thing as you've seen in PSP...I just don't know for sure. Hopefully you'll glean something out of this however.