I understand that the higher the letter, the yellower the stone. What controls the metallic/grey color? I notice some stones are super white and others are more metally looking. I hope I’m describing this clearly. Is that also based on color or something else like cut?
While the cut quality/angles may be modified to enhance color, the cut itself does not define the color.
If you notice when you get lower in the color range, stones can actually have a yellow or brown hue to them. I personally think the brown stones are prettier, but then again I don't care for the truly yellow stones either. This is just preference, and not to say brown > yellow.
Same deal with grey undertones. You could start to see them in colorless stones; however, if the color is enough they will be considered a fancy color.
Don't forget that lighting and camera setups can vary, even if they are apparently all set up to the same standard by a vendor, which can make stones look different to how they look in real life (annoyingly).