Copper is a first-row transition metal that produces a wide variety of colors in its compounds and complexes. Copper chemistry is interesting and easy, making it optimal for chemistry beginners. One such exercise involves the creation of every color in the rainbow using copper. In my example below, the only starting copper chemical was copper(II) chloride.
Red: Copper metal is reddish. In the photo, I took copper(II) chloride solution and reacted it with zinc metal to produce the reddish copper. I also added some ascorbic acid to the solution after the reaction was completed to absorb dissolved oxygen and prevent the copper from oxidizing to black copper(II) oxide. A red coloration is also obtained through copper(I) oxide. When glucose is heated with Benedict's reagent, red copper(I) oxide is produced. Another copper complex is deep red, and preparation instructions are found here.
Orange: Copper(I) oxide has the unique ability of forming a range of colors. The exact coloration depends on the rapidity of the compound's formation. When slowly prepared such as in the above reaction of glucose and Benedict's reagent, copper(I) oxide is red. The orange coloration in the picture was obtained by reacting ascorbic acid with copper(II) chloride to precipitate white copper(I) chloride. Sodium carbonate is added, and the orange compound precipitates.
Yellow: Tetrachlorocuprate(II) ion has a yellow coloration. When a small amount of copper(II) chloride is reacted with hydrochloric acid, a yellow solution (in the photo) is formed. Reaction of sodium hydroxide with copper(I) chloride precipitate forms yellow copper(I) oxide.
Green: A mixture of blue copper(II) aqua complex and yellow copper(II) chloro complex produces this deep green solution. I made it by dissolving a significant amount of copper(II) chloride in hydrochloric acid. When copper(II) chloride is crystallized from this solution, it is a green solid.
Blue: A dilute solution of copper(II) chloride (or sulfate or acetate) in water produces the sky-blue coloration of copper(II) aqua ions. Copper sulfate has a blue coloration when solid.
Violet: Tetramminecopper(II) solution is often violet. Copper(II) chloride (or carbonate or sulfate or acetate or oxide, provided that they are not strongly heated) is dissolved in aqueous ammonia. The precipitate of copper(II) hydroxide dissolves in the ammonia, forming a deep purplish solution of cuprammonium chloride.
White: Copper(I) chloride, produced when copper(II) chloride is reacted with ascorbic acid, is white until it gets oxidized by air.
Black: When copper(II) chloride is reacted with sodium hydroxide and heated to boiling, the unstable blue copper(II) hydroxide decomposes to black copper(II) oxide in solution, despite being surrounded by water.