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Thorium, a predominant actinide in the Earth's crust, presents significant environmental and health risks due to its radioactive nature. These risks are particularly pronounced during the mining and processing of monazite for rare earth elements (REEs), which contain substantial thorium concentrations. Current instrumental analysis methods for thorium, offer high accuracy but require laborious sample preparations and expensive instruments, making them unsuitable for on‐site analysis. Herein, we present a class of color‐tunable luminescent lanthanide‐based metal–organic frameworks (Ln‐MOFs) as fluorochromic sensors for Th4+ cations. Utilizing a heterobimetallic Eu3+/Tb3+ doping strategy, the luminescence colors of EuxTb1‐x‐BDC‐OH can be finely tuned from red, to orange, and to green. More intriguingly, the higher Lewis acidity of Th4+ facilitates the transformation of EuxTb1‐x‐BDC‐OH into a UiO‐type Th‐MOF via a dissolution‐recrystallization mechanism. This process results in a gradual reduction of characteristic Ln3+ emissions and the emergence of blue color ligand‐based fluorescence, thereby leading to selective fluorochromic responses with increasing Th4+ concentrations and enabling visible detection of Th4+ cations. Additionally, a custom‐built portable optoelectronic device is fabricated, which directly converts luminescence colors into red‐green‐blue (RGB) values. This device enables easy quantification of Th4+ concentrations without the need for complex instrumentation. [ABSTRACT FROM AUTHOR]