FeNCS2+ is colored complex ion so that it would absorb in the region 400 - 700 nm (visible region). For any colored compound, lambda maximum is set at which the compound absorbs maximum energy of light, send on this compound. Lambda maximum is different for different compounds. Solution FeNCS2+ is colored complex ion so that it would absorb in the region 400 - 700 nm (visible region). For any colored compound, lambda maximum is set at which the compound absorbs maximum energy of light, send on this compound. Lambda maximum is different for different compounds..