most aircraft's bellies are kept white or light colored to facilitate grounds checks.
occasionally, an airline will give one or a few of its planes a special paint job, in honor or an event or just as plain advertising. it is pretty near impossible to identify a commercial aircraft by looking at its belly only.
if you're looking at the color too, its a little better, you can find out which airline flies what and look for the airline logo on the tail. Certain aircraft have certain colors at certain places such as the airbus 320 series where the flap actuator fairing's tips are colored red.
though color and belly shape can help, the shape of the tail, wings/winglets/wing-tip fences, and number of engines along with the general silhouette and landing gear are other things you should look for if you want to ID an aircraft no matter where it is.