Do you mean a dash indicator light or the ambient lighting itself? Personally, I think that a cockpit with ambient lighting that adjusts based on autopilot status would be a much better indicator. When the autopilot is engaged and everything is copacetic, the lighting in the entire cockpit could be in a dim red color. As the autopilot gets less confident in the situation, the ambient lighting could get more yellow, with an increase in brightness.
The autopilot losing enough confidence and wanting to be turned off should blink between two colors and sound an audible alarm until the pilot presses a button to acknowledge the autopilot wants to turn itself off.
One interesting detail is that accelerating an A330 to M0.90 would probably not significantly damage the aircraft, if at all. For a unreliable airspeed issue, 85% thrust with 5 degree nose high attitude would keep the jet in the air. Keeping the nose much higher than 5 degrees risks a deep stall, which can easily result in a fatal crash.
There's a similar concept at play for medical devices. One part of IEC60601 specifies three alarm system priorities, and the colors get scarier, the visual indicators flash faster, and the tones get more urgent the higher priority the alarm is.
The autopilot losing enough confidence and wanting to be turned off should blink between two colors and sound an audible alarm until the pilot presses a button to acknowledge the autopilot wants to turn itself off.