If I understand correctly, all a/c altimeters, be it old capsule type or CADC/ADC driven are calibrated to ISA conditions.
If you are flying in temperatures above ISA, air being less dense, your actual true altitude is higher for the same altimeter setting. The mates around you will also be enjoying the same situation in terms of terrain clearance, so we are not worried when flying in ISA+ conditions provided everyone is on the same alt setting.
But if the temperature is below ISA the opposite occurs. The Jepps reference manual has a page for temp correction below ISA.
We know every millibar equates to approx. 30' at lower levels, the question is for every degree C below ISA, what is the altitude correction wrt true alt. From the Jepps it appears to be quite small until when it get pretty cold then the error become significant Can't remember off hand though.
Don't really operate in those conditions but a kind Canadian pilot with us liked to share this knowledge quite often.
Way to go.