Suddenly lots of response to the subject! Hmm. Well, it is a bit limiting to say red and blue, as the spectrum is fluid. What we do know is that chlorophyll requires light from the blue end (high energy, small wavelengths) and the red end (low energy, longer wavelengths). This is all relative, as we are talking about the elctromagnetic spectrum which ranges from gamma to infrared to radio-waves. The PAR part of the spectrum is light visible to humans. This relates to our view of the world. Many animals see beyond these wavelengths (380nm to about 700nm is human vision), with birds seeing well into the untraviotet, as well as insects, fish, crustations, and so on. Over about 750nm we call is simply heat!
In the attached diagram you can see that the green to orange range is pretty much inactive for photosynthesis. Land plants have chlorophyll a and b. Carotenoids vary from plant to plant. Now, in order to enjoy the myriad colours of our plants we require the entire spectrum in the lighting source, remember, human vision is based on red-green-blue sensitivity and our brains do the rest.
Another factor to consider is the light output of specific LEDs. Blues produce much more light energy than red consuming the same current. Most arrays for plants are 60% red to only 40% blue to compensate for this. Also, red light triggers the blooming sequence in most plants and needs to be properly administered to steer the blooming (think of seasonal pot plants).
As the aesthetic is, for myself, a deciding factor, lamps I've designed contain white LEDs to compensate for the otherwise missing spectrum.
I hope this is not getting to complicated.