Growing vegetables in greenhouses extends crop production seasons in northern latitudes, but the greenhouse environment is far from ideal for providing plants with optimal photosynthetic light.

In northern latitudes, low light is considered the most limiting environmental factor of greenhouse vegetable production.

For example  low light levels can result in formation of loose heads and low fresh weight of lettuce, and are also attributed to reduced anthocyanin (pigments that act as potent antioxidants and antibacterial agents) 

content of some lettuce cultivars. Regardless of leaf color or morphology, all varieties grown under low ambient greenhouse light developed darker red foliage and had increased relative chlorophyll

content after exposure to EOP (end of production) SL lighting from monochromatic red or blue or 50:50 red:blue LEDs

https://www.lightera.net/line-strip-grow-light-60w-37-5w-high-efficiency-tube-light-for-vertical-greenhouse_p74.html

End-of-production supplemental lighting demonstrates an alternative and cost-effective pre-harvest greenhouse practice for lettuce production in northern latitudes. The practice can allow growers to

manipulate leaf color in 5 to 7 days, thus increasing aesthetic appeal, quality, and market value without negatively affecting growth or morphology. There are economic benefits of growers, too, as they

can reduce the number of LEDs needed to supply supplemental lighting for entire crop production cycles. 

#Lightera Greenhouse Top Light High Power Full Spectrum G3-2in1 680W 3.0umol/J  LED Grow Light fixture for Horticulture Solution to increase anthocyanin synthesis and pigmentation of their lettuce crops.

https://www.lightera.net/greenhouse-top-light-high-power-full-spectrum-g3-2in1-680w-3-0umol-j-led-grow-light-fixture-for-horticulture-solution_p68.html