When you pour potassium dichromate solution into lead (II) acetate solution, yellow precipitate forms. This yellow compound is lead (II) dichromate which was used as a pigment for paints.
Most of the older paintings contain this salt and because of the sulfides they become darker. Nowadays, because of the toxicity of this lead compound it is replaced by cadmium sulfide which is less toxic and doesn't become darker in the air.