Although the quality of the cut is the most important factor in the beauty of a diamond, you will also want to take into account the color shade of the diamond. Cushion shapes show more color than a round brilliant, but their brilliance and dispersion help to mask the color.
For a cushion cut diamond set in a white gold engagement ring, opt for a color shade of H or higher. If you are on a budget, you can switch to an I or even J colored diamond while still maintaining a white appearance.
You can afford to choose a shade below (I or J) when the setting of your ring is in rose or yellow gold. Indeed, on these settings, an I or J colored diamond will always appear white. K colored cushions will have a slight tint, which can work well for vintage styles.