It could be one of several things that caused the discoloration of your radiator. First, if the paint gets too soft when the radiator is hot, it will collect dirt (dust and tobacco smoke) from the air. Second, if you painted the bare metal of the radiator, some rusting could be occurring and causing the yellowing. Last, if the paint used was solvent-based the heat of the radiator could make the paint turn yellow.
I suggest you repaint the radiator using a good quality pure acrylic matt or satin paint that is recommended for use on radiators. There are several premium brands on the market available from the large DIY retailers or from a good independent store.