see the color of (one's) money
To view the money that one will pay with in order to verify that they have it. Sure I trust you—but I still want to see the color of your money so I know you're good for it.
see the ˌcolour of somebody’s ˈmoney
(British English) (American English see the ˌcolor of somebody’s ˈmoney) (informal) make sure that somebody has enough money to pay you, especially if you think they might not have it: I want to see the colour of his money before I start doing such a dangerous job for him.