Thanks for the reply. But I guess I'm too newbie to understand expression script . Anyway, I found another way to cheat and the following is what I did:

create a emitter 1st > Color > Add Per Particle Attribute > rgbPP > Create Ramp > assign birthTime in Input V > Edit Ramp, delete the blue color from the default ramp and set the Interpolation to None. Finally add some Noise and Noise Freq to the Ramp for randomness.
:bgreen: