Building or passing storms. Be there (safe and dry!) before, during and after.
Also remember, "red sky at night, shepherd's delight; red sky in the morning, shepherd's warning." [~Oxford Dictionary of Proverbs]
Atmospheric smog from big cities will interfere with the clarity and nuances of colour in sunsets (not so often in sunrise). Go far, far away from big cities, the more remote the better.
A lot of the time it will be down to pot-luck: you being there with the camera ready for the unexpected. If nothing happens...oh well, there's always tomorrow...