When does yellow become a positive colour factor?

Most diamonds used in jewellery range from colourless to light yellow and are graded on a scale from D (colourless) to Z (light yellow). Those with less colour, or closer to colourless, are generally more valuable. Diamonds with deeper shades of yellow (more colour than Z) are graded differently and given a fancy-colour grade. For these coloured diamonds, a more vibrant colour typically means higher value.