RS343 -RBG video Signals need a Flag when R or B signals are missing

Hi all,

Please your help

Have RS343 -RBG video Signals, need a Flag when R or B signals are missing

Note:The Green video signal contain the synchronization and blanking pulses.

        The Red and Blue signals contain the blanking signal

        -need to produce a white image with gray shades when only the green signal is provided

        -  Any idea circuit,

         -  link to RS343 standard 

Thanks-Dov 1-516-487 5314