Why are newly grown leaves red? After a period of heavy rain, several trees in my garden will put out an impressive burst of new leaves, with an incredible vibrant red colour, almost the colour of port wine. The new leaves will then slowly turn green. I understand that the leaves turn green once they start producing chloroplasts and chlorophyll, but what is responsible for this initial red colour that is so very vibrant? And what biological role does it play?

It is anthocyanins that are produced to protect the developing photosystems (chlorophylls) from being damaged by sunlight.

As you have noted, it dissipates as the photosystems become adapted to the ambient light levels. Were you able to shade your trees for several weeks, you would likely also see their leaves turn red right after you removed the shade (easily done with trees in pots). Again, it is anthocyanin positioned in the leaf in a layer above the chloroplasts, but it will not be as vibrantly red because of the green chloroplasts below.