When copper(II) carbonate (CuCO₃) is heated, it decomposes into black copper(II) oxide (CuO), carbon dioxide (CO₂), and water (H₂O).

The reaction is:

CuCO₃(s) → CuO(s) + CO₂(g) + H₂O(g)

So, the color change you observe is from green copper(II) carbonate to black copper(II) oxide.