Off the top of my head, I can think of 3 ways to do this.
1. Create a color composition. Put purple solid on the bottom half of the composition, a white on the top half. They need to overlap. Bring that precomp into your text comp. Place the text on a layer over color comp and change the color comps TrkMat to “alpha”. You will have to move the text around until it’s exactly where you want it. Then parent the color comp to the text. You will be able to move it anywhere after that and the colors will follow.
2. You could put your colors in the same composition with your text. Place the text on the top layer and change it’s Mode to Stencil Alpha. This would work okay, if it’s the only thing in the comp, but if you start using other layer modes, it could mess things up. Best thing would to make this it’s own precomp and bring the entire thing into your final.
3. Create your text and under layer styles choose gradient…Won’t give you a sharp line, but a nice smooth transition between colors.
Johnny Cuevas, Editor
http://www.thinkck.com