Hey Carpe,
Taught a little GS on this. The material always presented the red squares as maneuvering limits, based-on numbers you put in the box compared to the Boeing perf manual (stored in the FMC's) and obviously info from ADC's, etc. But it can also be a placard speed limit if lower.
The top yellow line was presented as "max. manuever speed to buffet." And yes, I am sure these values will change depending on whether 1.2 or 1.3 G is your selected perf. as has already been mentioned.
Hope the terminology is correct...I haven't taught a GS class in a couple months...that stuff disappears fast.