If I remeber my grade 12 physics right yes it is true. Light is made up off all the colors just in different proportions. What happens whewn we see the color of an object all the other colors are absorbed by the surfaceof the object and the color we se is the color that is reflected back. So because plants are green the reflect almost all green light back at us with out absorbing it. That is why it is possible to work in a dark cycle with green lights with out having any effect on your plants. The only question is how do you get into your grow room without leaking other light in, unless you have a double door system like in photography dark rooms.