Review the item:
At a garage sale, Clarence bought four gallons of exterior latex for only
ten dollars then, when his neighbors saw the nuclear
pink,
they collected 120 dollars to buy four gallons of primer and four
gallons of forest green paint.
When his neighbors saw the nuclear pink is a subordinate clause. A subordinate clause is an incomplete idea that you must connect to a main clause. They collected ... begins a main clause. When a subordinate clause introduces a main clause, you need a comma to connect the two sentence parts.