What causes temperature gauge in car to be cold?

If you are referring to the temperature reading on the gauge, then it's possibly a bad thermostat.

It will also read 'cold' until the engine has sufficiently warmed up.

Lack of fluid (water and/or antifreeze) in the radiator.

Low engine temperature (bad thermostat).

Low coolant level.

Bad gauge.

Wiring issue.

Coolant temperature sensor bad or not correctly grounded.

Problem in an electornic control module.