Leaves turn colour according to day length, temperature and soil moisture. A dry summer tends to make the leaves turn brown and fall early without the vivid colours and we’ve seen that happen this year. At the end of the growing season sugar production shuts down and the leaf stops producing chlorophyll. Sunlight kills the residual chlorophyll, so they lose their green pigment and their other pigments are unmasked and intesified.
Some trees have more red leaf pigment, others more yellow etc, which is why different varieties of trees have different autumn colours but they all need the sunlight to destroy the green for us to see the other colours. So to get good autumn colours we need warm sunny days and cool nights.
But you’re so right about the clear blue sky setting off the colours! 
Originally Posted by: Caz