See figure.
EASA learning objective 022.12.02.01.02 states: 'Explain master warning: colour of annunciator: red; nature of aural alerts: continuous; typical failure scenarios triggering the alert.'
EASA learning objective 022.12.05.01.01 states: 'Explain the purpose of an overspeed warning system (VMO/MMO pointer).'
EASA learning objective 022.12.05.01.02 states: 'State that for large aeroplanes, an aural warning must be associated to the overspeed warning if an electronic display is used (see AMC 25.11, paragraph 10.b(2), p. 2-GEN-22).'

EASA CS 25 gives the following definitions with regards to alerts required to draw the crews attention to an abnormal aircraft condition:

• Warning - requires immediate flight crew awareness and response (Red light or caption)
• Caution - requires immediate flight crew awareness and subsequent response (Amber light or caption)
• Advisory - requires flight crew awareness and may require subsequent response -

Master Warning - Many modern aircraft, such as the Airbus A320, have a red master warning light, situated in the pilots eyeline, which flashes as an attention getter to alert the pilots when a system warning is triggered. This is usually accompanied by an aural alert. If an aircraft is fitted with a master warning system then any critical situation that requires immediate flight crew awareness and response will trigger it.

Typical scenarios that would trigger a Master Warning are:

• Engine fire - this obviously requires immediate flight crew awareness and response so a warning is triggered
• Overspeed - if the aircraft is exceeding its limiting airspeed then this requires immediate flight crew awareness and response to prevent structural damage.

Brake temp overheats are not uncommon and while they require immediate flight crew awareness, they only need a subsequent response so are typically displayed as amber cautions.

Note: While most aircraft have Master Caution alerts not all have Master Warning systems and those that do may have differing operating criteria, so this question is less than ideal.