Red means stop as soon as it is safe to do so. Amber means get it looked at soon. Ignoring a red oil pressure or temperature light for a few miles can destroy an engine.
- Red oil pressure light: stop immediately and safely. Do not drive on. This is not a "low oil" warning, it means the oil is not circulating, and an engine will destroy itself in a short distance.
- Red temperature light or gauge in the red: stop, let it cool, do not open the coolant cap.
- Red battery light: the charging system has failed. You are running on the battery alone, so head somewhere useful immediately.
- Red brake warning: check the handbrake is off, then stop and get it looked at.
- Amber engine management: get it diagnosed soon. If it is flashing, that is more urgent and can damage the catalytic converter.
- Amber tyre pressure: check them at the next opportunity.
A car that still drives normally is not evidence that a red light is a fault with the sensor. That assumption is the single most expensive mistake in car ownership, and it is usually made about the oil pressure light.
Red lights are reserved for conditions that cause damage or a loss of control in a short time. The colour convention is consistent across manufacturers precisely so that no interpretation is needed.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.