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We can override the parameters in PxrSurface using the PxrLayer options. 

In this example, we set a green diffuse color in PxrSurface. Then we layer a PxrLayer node on top that has a cyan diffuse color. Note that one color covers the other, so we need to alter the Mixer node.

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We can combine layers using PxrLayerMixer. The mixer is where we decide where and how layers are mixed using masks. Below, patterns are connected to the Mask in the Mixer.

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