The modifiers on the Effect tab require unlimited access to colors and therefore are only available when editing True Color and Windows XP images.
Anchor points are used to define how a gradient is created. Anchor points can be turned on or off. Anchor points that are turned on can be set to either color A or B. Hold down the SHIFT key and click a button to toggle the anchor point on or off. Click a button that's "on" to toggle between color A and color B.
This Effect maps the gradient anchor points over the entire image. The color that any pixel is painted will be relative to the full graphic.
This Effect maps the gradient anchor points to the leftmost, topmost, rightmost and bottommost pixels in the current paint operation. The gradient colors are dynamically adjusted during the paint operation for most drawing tools.