Seems like as if you vertex shader consists solely of this one line...that's not what i meant. You have to provide a vertex and a fragment shader and the vertex shader has to have this colorinfo (or whatever name you choose) attribute in addition to the other attributes. You can modify one of the default shaders for this, i.e. unzip the jpct_shaders.zip in the lib directory of the distribution and pick a shader that fits. Which one that is, highly depends on your scene. If you want to be sure, you can simply pick defaultFragmentShader and defaultVertexShader. They include every feature that a scene in jPCT can use but if it doesn't, they might be slower than needed. Anyway, i would start with them. Add the colorinfo attribute to the vertex shader and do something with it. You most likely don't have to modify the fragment shader.
If you want to use vertex colors only without lighting or even textures, the shader can look a whole simpler than the default shaders. It depends, what you want to do with it...