Thanks Dave and Rogelio. Rogelio, I use a special filter for the color staining. I place the filter over the lower polarizer and 1/4 wave plate and pull the upper dic prism out as far as possible. I adjust the lower filter until I see a bright pink to blue gradient background. The filter resembles a crude phase stop. I cut a ring and a small solid circle out of cardboard and taped them to a microscope slide. There is a gap between the center circle and the outer ring. The effect only works on 10x and 20x objectives with this filter. The filter seems to interfere with the polarized light and mimics the effect of different compensators. See this link.