While you may think diamonds are clear, they are all graded on the basis of their colour. The colour scale starts with D, which is colourless and continues through to Z which has a light yellow hue. If a diamond is a saturated pink, blue, green or yellow, however, they are considered very valuable due to their rarity. Diamonds are graded for colour face down, through the pavilion.
Colour should be assessed under daylight or fluorescent light. Another factor that can affect colour is the diamond's proportions. A shallow diamond can look lighter than a well-proportioned diamond. Size also plays a part in the visibility of colour. The larger the diamond, the easier it is to see colour variations.