Here is a link to the section on symmetry. The section on polish follows. On larger diamonds, I prefer very good and up, but it depends on the reason for the grading. As for color, I think you should go see a variety of properly graded diamonds and decide what you need. You should see them dropped into settings in the color you will buy and then decide how they look face up.
Color is much less noticable when mounted.
Better cut diamonds also face up whiter. Go through the tutorial and try the cut adviser for guidance. Yes, I have read it and where it states GD or better and that SYM and Polish really won't effect the beauty of a diamond. The stones with good angle combinations show good firescope images. However, there are some AGS 0 cut stones that have less optimal combinations and that do not perform so well.