The most striking example of colourful citrus fruits is the blood orange. They have very intense pigmentation, both in their peel and pulp.
In other citrus fruits, the colour is determined by the accumulation of another pigment, lycopene. Unlike anthocyanins, which release the colour red when citrus fruits are subjected to frequent significant temperature fluctuations, Lycopene deteriorates with the lowering of temperature.
It is for this reason that citrus fruits such as Star Ruby grapefruit and Cara Cara orange have such intense colours, since they are grown in climatic zones that always experience relatively high temperatures.
The colouration of citrus fruits may also involve buds and flowers; this is the case for lemons. In lemons, anthocyanins stimulate the pigmentation of buds and flowers, and much more rarely act on the skin of the fruit. This is why it remains bright yellow without turning shades of orange or red.