Making one material change into another, but from the bottom up?

Let’s say I have a letter, “C.” I then want the color green to form from the bottom and rise to the top, filling the letter. Can somebody help me figure out a way to do this?

Animated texture using a movie file.

If you want to do precisely what you asked, i.e. make a color move across an object from a specific direction, you can do it simply using a Halo Lamp. It helps if the object is white to begin. Set the lamp color to whatever you desire, checking “square” for the halo cone might help. The light of the halo lamp is confined to the halo cone so animating the edge of the cone across the object will color the object progressively in whatever direction you wish.