Simply by wishing it to change colors, of course.
Have you scoured the help files for an answer? What plug-in are you using? Is it shot footage? Have you tried the "Hue Saturation" effect?
For future reference, a quick jaunt over to the community search area should be a good avenue to pursue before asking in the forums.
Change a smoker's color?
I must have misread that post. If it's not racial in nature, use any of the Color Correction filters that seem appropriate.
If you're using a particle system to create smokish effects, find the appropriate color controls.
There are so many different variables here that it is hard to give any sort of specifics. What is the smoke (footage you shot, stock footage, particle effect, etc.)? What sort of color change do you want? If none of the advice you've already received is helpful, please give us more details.
Since you seem to be very new to AE, I would highly recommend you start here first.