Witness paint

Also known as Sentry Paint. Witness paint is used to easily show if a nut or bolt has moved. Ideally it should be a strongly contrasting color, resilient, shouldn't come off easily and survive high temperatures in the engine compartment if it is used there.

Torque Seal is one recommendation for this purpose. It's available in a number of colors. Aircraft Spruce carries it. This is a low-cost safety item, with a tube costing about $3.

Torque Seal in Yellow

Related terms

SentryPaint