I am assuming that you extracted the six frustum planes from an existing view-projection matrix.

If that is the case and if your question was how to color all the fragments of the rendered model that fall inside of that frustum, then just do this:

1. transform the model-space vertices of the model by the model matrix and the camera's view-projection matrix as you would normally do

2. also transform the model-space vertices by the model matrix and the view-projection matrix describing your additional frustum

3. output the homogeneous vector from that 2. step as interpolated varying/out variable from your vertex shader to your fragment shader

4. in your fragment shader, color the fragment when their x, y and z coordinates all are within -w to w. This is basically what OpenGL does with your camera-transformed vertex positions written to gl_Position, just that it performs clipping based on that.

In the case where you really only have the six planes, simply only transform the model-space vertices only by the model matrix in step 2 and perform a point/plane distance test with all the six faces and output those six distances to the fragment shader as interpolated float values. Then, inside the fragment shader, test whether any of those distances is negative, in which case that fragment got effectively clipped by that plane.