Colours on a dull day. Notice them. Let them nudge your mood. ‘Dull’ comes with far too much implied negativity. As a benchmark it’s fine – ‘the light’s dull compared to yesterday’. As a valuation it’s learnt.
With everything you learn, ask who or what’s taught it. It might be neutral. It might be benign. It might benefit you. But it might have any number of negatives in its train.
The why of it
I think simply a response to a dull day during a dull month.
You only have your own reactions to what’s around you to argue with.
That, and the source of the reactions.