Well this is true but it is also very misleading, for example this is a statement often bandied around by white light companies and in no way means that green light is better for your plants.
So firstly lets define what a white light is right? A white light is just a mixture of all colours in varying amounts until it appears white (cool white, warm white) in some manner to us humans.
It goes without saying that green will also be used when mixed with other mono led colours as this is what white light is made up from.
For example most HPS spectra is primarily green through to orange with little or no blue and very little red 660nm.
If you look at the chart below you will see one of the market leading HPS brands spectral output and it does leave you wondering how the plants grow at all with so little blue?? Well this is because green light helps drive photosynthesis harder so even though a spectrum can have very low amounts of blue and red the photosynthesis is driven harder by the presence of green light.
We include varying amounts of green - orange in most of our lamps as we found in our own testing that final bloom stages without any green and amber resulted in lower quality end yields.