I hear that you‘re not supposed to air up a tire when It‘s cold so what do you do If a tire needs air and It‘s cold out?
You heard incorrectly. Tire pressures should always be adjusted ONLY when the tire is COLD. That is the only way to establish an accurate baseline. When the tire heats up (from the friction of driving) the air pressure increases. Unless you knew what the cold pressure was before the tire heated up you have no way of knowing how much pressure change there was. Therefore you have no way of knowing how much air you should add/subtract.
Don't know where you heard that from. You determine the air pressure in your tires most accurately when the tire is cold and has not had much friction. Pressures are usually pretty close unless you are in a situation like a very hot day in Arizona , you have been driving for a long time on a black curvy road that is in the direct sun. Then you will get a reading substancially higher. If you need air, just add it. if you need to add air too often, you have a puncture probably if it never did that before. Air does escape a little bit, but not much unless the tire has cracks in the sidewall. Keep concious of the air in tires low air up front can cause understeer and in the back can cause oversteer, and make you spin out easily.
That is completely back-***-wards. The tires are supposed to be inflated cold. But cold is not referring to the temperature outside. It means the tire has not been driving for more than a mile before the pressure is checked and corrected. Air expands when heated, so hot tires will have higher pressures than cold tires.