Developing simple, fast and sensitive detection of low-level water in organic solvents is in high demand for academic and industrial applications. In this paper, a novel color-tunable lanthanide composite has been prepared by simply mixing 2,2':6',2"-terpyridine-4'-carboxylic acid (Hctpy) and Ln(3+) with a molar ratio of 3:1 in absolute ethanol at room temperature, in which the Ln(3+) ions are mainly coordinated with carboxyl moieties rather than with terpyridine moieties of Hctpy. In addition, the emission colors of the composite can also be tuned by water, ascribed to the luminescence quenching mechanism and the variation of Tb-to-Eu energy transfer efficiency. Based on these properties, a self-referencing and ratiometric luminescent water detector with the Eu/Tb molar ratio of 1:1 has been developed, showing proportional relationship between the intensity ratio of D-5(0) -> F-7(2) transition of Eu3+ to D-5(4) -> F-7(5) transition of Tb3+ (I-Eu/I-Tb) and the water concentration (0.01-0.5 vol%) in ethanol.