pure water (water was only an example to explain my goal) does darken with depth, the existence of other particals will influence and speed up how much light is absorbed though. proof of this can be found by taking a piece of pipe, capping both ends with a clear see-through material say glass, and fill with highly filtered water, or so you can't claim not clean enough, buy some hydrogen and oxygen and make your own water =P the degree of absorbtion is different, but it still happens

mark, the technic in your post is using color change based on the elevation of the part of the model, which is basicly the same as attaching a color map to fake it, no offence just trying to avoid that -_- 

wonder if a shader plugin can be wrote to measure the length of rays (after taking into account raytracing effects)  as they travel "inside" of an object, and provide a color gradient for it to color the model that way?