Near-infrared light sits just past the red end of the rainbow, where our eyes stop seeing. Photobiomodulation research usually draws on two bands: red light, around 600 to 700 nanometers, and near-infrared light, around 780 to 1100 nanometers (de Freitas & Hamblin, 2016).
The reason is depth. Red and near-infrared light travel farther into tissue than other wavelengths, because less of it is scattered or absorbed along the way. The band in between, roughly 700 to 780 nanometers, has been found rather ineffective, because it falls in a trough of what cytochrome c oxidase absorbs (de Freitas & Hamblin, 2016).
Why it matters
Color is not decoration in this work. It decides how far light can reach and what can absorb it when it gets there. As well, near-infrared light is invisible, so a device can be giving it off without looking bright.
References
de Freitas, L. F., & Hamblin, M. R. (2016). Proposed mechanisms of photobiomodulation or low-level light therapy. IEEE Journal of Selected Topics in Quantum Electronics, 22(3), 348–364. https://doi.org/10.1109/JSTQE.2016.2561201