Supplemental lighting is proven to increase transplant growth and quality in vegetable nursery greenhouses. To evaluate plant responses to supplemental LED light quality, tomato seedlings ('Komeett') were grown in a greenhouse (Tucson, AZ, USA) until the second true leaf stage with 55.5±1.4 μmol m-2 s-1 photosynthetic photon flux of supplemental LED lighting (18-hour photoperiod). Treatments consisted of different red:blue photon flux ratios (1) 100% red:0% blue, (2) 96% red:4% blue, (3) 84% red:16% blue and a control without supplemental lighting. These ratios were evaluated under low and high daily solar light integrals (DLI) (8.9±0.9 and 19.4±1.9 mol m-2 d-1, respectively) created by different shade screens deployed in the greenhouse. Growth and morphological parameters including dry shoot mass, leaf count, stem diameter, hypocotyl length, leaf area, and chlorophyll concentration indicated the benefit of supplemental light, especially under low DLI, but there were no significant differences among different red:blue ratios regardless of DLI. The seedlings also exhibited the same high photosynthetic capacity measured under 1000 μmol m-2 s-1 PPF, ambient temperature and CO2 concentration regardless of the red:blue ratios. From this preliminary study it seems that for 'Komeett' tomato seedlings grown in greenhouse, use of 100% red LED supplemental lighting was sufficient and no additional blue light was required regardless of DLI.