ContrastBrush(id, inc[, repeat])
inc argument is set to True, the contrast is
enhanced. If it is set to False, the contrast is reduced. The optional
argument repeat can be used to apply the effect to the brush multiple
times. This is useful if you want to create sharper contrasts.
Note that if id specifies a palette brush, ContrastBrush() will
just apply the contrast to the palette colors which makes this function
really fast when used with palette brushes.
True to increase contrast, False to decrease contrast