Preserving colour of vegetables in Chinese hakka noodles
It's quite a simple technique actually, called blanching. You just need to:


Dip the vegetables in boiling water for a short time (10-20 seconds)
Immediately transfer them to ice-water (or cold/running water, if you don't have ice), to stop them from cooking with the carry-over heat. (This is also called shocking), and seperate.
Later, just before serving, combine and heat the vegetables back to the serving temperature.