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Supplemental blue/red lighting accelerated fruit coloring and promoted lycopene synthesis in tomato fruits. Potassium (K) is the most enriched cation in tomato fruits, and its fertigation improved tomato yield and fruit color. However, the effects of supplemental lighting on K uptake and transport by tomatoes and whether supplemental lighting accelerates fruit coloring through enhancing K uptake and transport are still unclear. We investigated the effects of supplemental light-emitting diode (LED) lighting (SL; 100% red, 100% blue; 75% red combined 25% blue) on K uptake in roots and transport in the fruits as well as the fruit coloring of tomatoes (Micro-Tom) grown in an experimental greenhouse in hydroponics. The use of red SL or red combined blue SL enhanced K uptake and K accumulation as well as carotenoid (phytoene, lycopene, γ-carotene, and β-carotene) content in fruits by increasing photosynthesis, plant growth, and fruit weight. The genes related to ethylene signaling were upregulated by red SL. Quantitative real-time PCR (qRT-PCR) results showed that K transporter genes (<i>SlHAK</i>s) are differentially expressed during fruit development and ripening. The highest-expressed gene was <i>SlHAK10</i> when fruit reached breaker and ripening. <i>SlHAK3</i> and <i>SlHAK19</i> were highly expressed at breaker, and <i>SlHAK18</i> was highly expressed at ripening. These might be related to the formation of tomato fruit ripening and quality. <i>SlHAK4</i>, <i>SlHAK6, SlHAK8,</i> and <i>SlHAK9</i> were significantly downregulated with fruit ripening and induced by low K. The expression level of <i>SlHAK6, SlHAK10</i>, <i>SlHAK15,</i> and <i>SlHAK19</i> were significantly increased by blue SL or red combined blue SL during breaker and ripening. Blue SL or red combined blue SL increased content of phytoene, β-carotene, α-carotene, and γ-carotene and accelerated fruit coloring by enhancing K uptake in roots and transport in fruits during fruit ripening. This was consistent with the expression level of <i>SlHAK6</i>, <i>SlHAK10</i>, <i>SlHAK15</i>, and <i>SlHAK19</i> during fruit development and ripening. The key genes of photoreceptors, light signaling transcript factors as well as abscisic acid (ABA) transduction induced by blue SL or red combined blue SL were consistent with the upregulated genes of <i>SlHAK6</i>, <i>SlHAK10</i>, <i>SlHAK15,</i> and <i>SlHAK19</i> under blue SL and red combined blue SL. The K transport in tomato fruits might be mediated by light signaling and ABA signaling transduction. These results provide valuable information for fruit quality control and the light regulating mechanism of K transport and fruit coloring in tomatoes.