The pressure altimeter is calibrated to the standard atmosphere. If the ambient temperature is below the standard atmosphere temperature for a particular altitude, the altimeter will over-read. Hence temperature below standard is the dangerous case.
ICAO Doc 8168:
"The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4 per cent height increase for every 10degC below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above -15degC."