One easy adjustment is to have PyFerret automatically draw thin outlines around its polygons, effectively covering up the thin white lines (grid pattern) that result from antialiasing, which blends into the background color (typically white).
From the PyFerret documentation:
SET WINDOW /OUTLINE=
When drawing color-filled rectangles and other polygons, this option allows one to specify the thickness of a outline to be drawn around each polygon. The color of the outline will be the same as