If you were making a product of artificial "Grow-Lites" for plants, what color light should they emit so that plants can do photosynthesis (make sugars) most effectively?

-blue and red light

Explanation:

Plants produce sugars or carbohydrates during the process of photosynthesis. They absorb light energy from the electromagnetic spectrum with pigments within the thylakoid membrane, like chlorophyll a, chlorophyll b.

Chlorophylls are made of ringed molecules chlorine, a hydrogenated form of porphyrin with a magnesium ion bonded to four atoms of nitrogen. Chlorophyll a shows the most absorption of red light  (642 nm) and blue light (372 nm); while chlorophyll b shows the most absorption at 626 nm and 392 nm.  

Different types of chlorophyll sidechains change the molecules' absorption ranges; A's methyl group is bound at carbon 7,  B's aldehyde (CHO) ring is bound at carbon 7.  Both absorb light from orange-red and violet-blue wavelengths. As such, the best light wavelengths for photosynthesis are within the blue and red wavelengths (425–450 nm) and (600–700 nm).