The most common type of grow-light bulb is the T5 & T8 LED Tube Light. The bulb is most often found in (more or less) 6,500 Kelvin, which is the color temperature of the light we see at daylight. Traditionally, blue lights would be used to provide plants with this necessary wavelength. Plants undergo vegetative growth during daylight hours, so this light is beneficial to plants during the vegetative stage of growth.
Bulbs that are ~2,700 Kelvin are beneficial to plants in the flowering and fruiting stages of growth. This is the color temperature during sunrise hours of the day. Bulbs that are 2,700 to 2,700 Kelvin are traditionally warmer in color, with more red and orange tones. The red lights used by some growers would deliver this color-temperature to plants.
Both the 2,700 and 6,500 Kelvin lights are available as white LEDs, so there is no need to use red or blue lights anymore. We recommend using fixtures that hold four bulbs, and alternating the 2,700 and 6,500 Kelvin bulbs in the fixture. This will ensure that plants receive the benefits of both.