There are two reasons. One, plants grow better under red light. Red light has the highest efficiency for photosynthesis in plants, that why LED grow lights are tuned for certain wavelengths. Plants use blue light to know what direction to grow.

And for a certain category of tints or dyes that are fluorescent, additional red light.

Luminescent solar collectors have an embedded dye with molecules that fluoresce, that give off even more red light.

Example: http://www.soliculture.com/lumo-technology/