This study focused on the effects of including a mixture of Hermetia illucens (HI) and Tenebrio molitor (TM) meals on feed colour, growth performance, carcass yield, somatic indexes, nutrient digestibility, intestinal microbiota, histomorphometry, and fillet quality of rainbow trout (Oncorhynchus mykiss). Seven diets were formulated: one control diet (CTRL) containing 15% fishmeal, two diets where the fishmeal was replaced with 100% HI (HI100) or TM (TM100) meal, and four diets containing increasing levels of a mix (1:1) of HI and TM meals, replacing 25% (MIX25), 50% (MIX50), 75% (MIX75) and 100% (MIX100) of the fishmeal. A total of 378 fish (average weight: 126 ± 1.71 g (g)) were randomly distributed in 21 experimental tanks (3 replicates/treatment and 18 fish/replicate) and fed for 84 days. A digestibility trial was also conducted to assess the apparent nutrient digestibility coefficients of the diets. Diet significantly affected pellet colour (L*, a*, b*; p < 0.001), with CTRL and MIX25 being the lightest and HI100 and MIX75 the darkest. Fillet colour, pH_24h, and drip loss were significantly affected by diet (p < 0.001). The pH_24h parameter, reflecting stress and freshness, was lowest in MIX25 and highest in MIX75, suggesting a synergistic effect and potential impact on shelf life. No significant differences were found in growth performance, carcass yield, somatic indexes, fillet chemical composition, intestinal microbiota, and the histopathology of internal organs among the experimental diets (p > 0.05), nor in nutrient digestibility. This study confirms that HI and TM meals, alone or combined, are viable alternatives in low-fishmeal diets for rainbow trout.