Amber bands / manoeuvre margins are affected from Altitude, CoG and weight.
LNAV protection, thats the computers calculating excess thrust and giving the authority for LNAV to bank more.
HDG select, doesn't do that for you which means that with a higher AoB selected and a not enough thrust you can decelerate in the amber band (where indeed, your manoeuvre margins will be more limited). Which confirms that even if you fly outside of the amber band does NOT mean that you have the required thrust to complete the manoeuvre.
I remember an article from Boeing explaining how at low speeds (ie 10,000) the amber/red bands are based on AoA & at higher altitudes/Mach numbers, the FMC bases the margins to either the stickshaker or initial buffet whichever is higher.
1.3g maneuver capability to low speed buffet (or an alternative approved maneuver capability as preset by maintenance)
So my wild guess is that
maybe they use full manoeuvre capability for reference to stickshaker and manoeuvre capability for initial buffet?
I stand to be corrected
