Why do suppliers charge more for the same item in a different color?

I am getting quotes from suppliers for this plastic product. Their stock product comes in white orblue. I am asking to get the product made in black. Black is a wildly popular color for this specific product but every supplier I’ve spoken to, says they will need to charge a hefty price per unit.

Can someone help me understand what goes on in the manufacturing process that just changing just the color of an item costs so much? I’m not asking for a new mold or any design modifications. Is it because of the price of the “paint”?