Copper forms both divalent (cupric) and monovalent (cuprous) compounds. Since most of the cuprous compounds are insoluble, they are relatively easy to prepare.
Copper(I) oxide: See here for the red form: Production To make the yellow form, react sodium hydroxide with copper(I) chloride. It will turn bright yellow. Another potential way to make the yellow form is to electrolyze concentrated salt water with a pure copper object as the anode. The anode initially turns white as copper(I) chloride is produced, then turns yellow as the NaOH from the cathode reacts with it. This may not make pure copper(I) oxide, which is susceptible to oxidation by any oxygen produced at the anode as well as air.
Copper(I) chloride: React copper(II) chloride (Production) with either ascorbic acid or sodium metabisulfite to form a white precipitate of copper(I) chloride.
Copper(I) bromide: React copper(II) sulfate with an equal amount of sodium bromide. Add ascorbic acid or sodium metabisulfite. White copper(I) bromide will precipitate.
Copper(I) iodide: React copper(II) sulfate and sodium iodide. A dark brown mixture of iodine and copper(I) iodide will form. Add ascorbic acid to reduce the iodine to water-soluble iodide and only a white precipitate should remain. You can also react copper sulfate and a mixture of tincture of iodine and excess ascorbic acid. A white precipitate of copper(I) iodide will form.
I reacted copper(II) chloride with a mixture of tincture of iodine and ascorbic acid. Several reactions occurred at once: reduction of copper(II) chloride to copper(I) chloride by ascorbic acid, reaction of copper(I) chloride with copper(II) chloride to form the dark brown complex seen here, reaction of copper(II) chloride with sodium iodide to produce brown iodine and colorless copper(II) iodide, and reaction of iodine with ascorbic acid to form colorless iodide. The end result, after all products were mixed together, was a light brown precipitate.