Formation of green precipitation whilst adding potassium fluoride to aqueous solution of copper(II) sulfate
$\ce{CuSO4}$ exists as $\ce{[Cu(H2O)4]SO4}$ in solution. It is blue in colour due to the presence of $\ce{[Cu(H2O)4]^{2+}}$ ions.

Now if aqueous $\ce{KF}$ is added, the solution turns green due to formation of complexes $\ce{[CuF4]^2-}$ and $\ce{[CuF6]^4-}$(Initially copper(II) fluoride($\ce{CuF2}$) is formed but it formed complexe in present of water). The complete reaction is given by:

$$\ce{[Cu(H2O)4]SO4 + 4KF -&gt; K2[CuF4] + K2SO4 + 4H2O}$$