Copper sulfate CuSO4 is typically a bright blue color when in its pentahydrate form CuSO45H2O , commonly known as blue vitriol or bluestone. The blue color of copper sulfate is due to the presence of the copper II ion Cu2+ in the compound. The copper II ion absorbs certain wavelengths of light, particularly in the red region of the visible spectrum, and reflects or transmits the remaining wavelengths, which results in the blue color that we perceive.