Colour of $\ce{Cu^2+}$ is blue. Then why is the colour of $\ce{CuCl2}$ green and $\ce{CuO}$ black? Because both of these compounds have $\ce{Cu^2+}$.
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3$\begingroup$ Maybe $\ce{CuO}$ and $\ce{CuCl2}$ are not made of ions. Anyway $\ce{CuCl2}$ is not green, but brownish yellow. Only its hydrate $\ce{CuCl2·2H2O}$ is green. How do you know that $\ce{Cu^{2+}}$ is blue ? It may be blue in solution, or in the presence of water. $\endgroup$– MauriceMay 26 at 8:43
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1 Answer
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$\ce{[CuCl4]^2-(aq)}$ is even yellow. $\ce{Cl-}$ ligands attached to $\ce{Cu^2+}$ shift color toward green and yellow.
Solids are special. They may follow color of solutions, if structure is not far different. Or, the structure and related color can be very different.
There is also big difference if the ion in solid is hydrated or not. Anhydrous copper(II) sulphate is near white while its pentahydrate is blue.
Why should copper(II) oxide be blue, if copper ions are not hydrated? Also, the oxide structure and nature of interaction of copper and oxygen in the oxide are very different to $\ce{Cu^2+(aq)}$?