The fastest way to get cold in the mountains is to get wet, and the most common way to get wet is your own sweat. The whole layering system exists to stop that.
01Base: move the moisture
The layer on your skin has one job, which is to pull sweat away before it cools you. Merino does this well and stays warm when damp. Cotton does the opposite, so leave it at home.
02Mid: trap still air
Fleece and down are warm because they hold air that does not move. Carry a midlayer you can take off in one motion. You should start every climb feeling slightly cold.
03Shell: keep the weather out
A shell blocks wind and water and lets vapour out. Open the pit zips before you overheat, not after.
Be bold, start cold. If you are comfortable in the first five minutes, you are wearing too much.
At every stop longer than two minutes, add a layer before you feel cold. It takes far more energy to warm up than to stay warm.