The easiest metals to grade color in are platinum and rhodium plate over platinum or white gold. Platinum has a very slight tint of gray and rhodium is a very white metal, both of which allow a relatively neutral background to grade color. The problem here is the limited view into the pavilion. Even these white metals will detract color and if the rest of the ring is yellow gold, this will likely detract yellow in an off-color diamond or add yellow to a colorless diamond. In some bezel style settings, the pavilion is completely enclosed which leaves only the crown to grade color with. This of course lowers the odds even further in achieving a near-accurate color grade.