A constraint can force inventiveness that unlimited freedom never demanded.

Unlimited freedom gives you no reason to leave the first solution, because nothing forces you off it. A constraint removes the easy path and makes you search regions you would never have visited voluntarily. That is why limits so often produce the work people remember.

When you feel stuck with too many options, impose an arbitrary restriction. One colour, three sentences, no budget, a fixed hour. Choose something you would not have chosen, and see what the restriction makes you invent to get around it.