Preferably a blue-violet light with wavelengths of 440nm or a red one with 660nm wavelengths.
LEDs are apparently the most efficient for photosynthesis, and common,y used in farms. (see site)
The process of photosynthesis requires luminous energy to "activate" electrons that are situated in one of the chloroplast's compound, the chlorophyll.
These electrons are crucial for the next steps of photosynthesis (ie. the
But, the chlorophyll isn't sensitive to the wavelengths of all seven visible light rays.
You can visualize the variation of its absorption in this graph:
It mostly absorbs 440nm radiations from blue to violet light and 660 nm radiations from red light.
Preferably, you should use LEDs that emit light at one of of those two wavelengths.
Bonus: this also explains why leaves are green: the big fraction of green light with 500 to 550nm wavelengths is not absorbed by the leaf's chlorophyll- the leaf reflects those green rays and appears green!