The utility of Tb3+:Mn4+:Na4Mg(WO4)(3) phosphors as ratiometric luminescent thermometers in the 200-450 K temperature range is demonstrated. Targeted substitution of Tb3+ for Na+ and Mn4+ for Mg2+ yields phosphors of formula Na4-12xTb4xMg1-2yMny(WO4)(3). UV excitation of the Na4Mg(WO4)(3) host results in green emission from terbium (544 nm) and red emission from manganese (682 nm). Differential thermal quenching of these emissions renders Na3.976Tb0.008Mg0.990Mn0.005 (WO4)(3) thermochromic between 78 and 450 K, with an orange-to-green color change that is particularly noticeable above 200 K. As a result, this phosphor serves as a ratiometric and colorimetric thermometer between 200 and 450 K. Quantitative assessment of its performance yields a maximum relative sensitivity of 2.5 x 10(-2) K-1 at 375 K, along with a resolution of 0.5 K and a repeatability of 98%. These findings highlight the versatility of Na4Mg(WO4)(3) as a platform to realize luminescent thermometers via targeted aliovalent substitutions.