Talkbacks and Barberpoles
The "gray talkback" is an example of the indicators which were mounted around some of the instrument panels. Each talkback consisted of a small window with a black and white stripe pattern behind it - the "barber pole" often referred to during the mission. A gray flag could move in front of the barber pole stripes to indicate the status of a particular system. The gray flag would be driven by a control signal from the system in question and it told the crew what was going on in that system, therefore the indicator was called a talkback. Gray was essentially a normal or "doing nothing" indication. A barber pole in the talkback usually meant an abnormal or transient status.
For those who wonder at the term "barber pole", it comes from the red-and-white-striped pole that barbers display outside their establishments to make them identifiable to their clients. It harks back to a time when barbers indulged in dentistry and surgery.
A recurring problem in the Apollo program was that the RCS propellant isolation valves [essentially, shut-off valves] occasionally closed during periods of heavy vibration or shock, rendering that particular RCS thruster unusable. Because the Service Module and Lunar Module used similar components, the valves in the RCS systems of both spacecraft could be affected by this. Indeed, one of the first post-landing steps for the Lunar Module was to cycle these valves to ensure they were open. Fortunately, this problem usually was regarded as a nuisance, since it was easily remedied.
[edit] Apollo 15
The postflight mission report concluded that variations in the supply voltage combined with the launch vibration was most likely to blame for closing these magnetic latching valves. (ap15fj)