The Arnold Utility shader has an Object color mode, which assigns colors based on shape names. When we say “shapes”, we don’t mean Maya shape nodes; we mean the Arnold shapes.
C:\solidangle\mtoadeploy\2014\bin>kick -nodes | find "shape" box shape cone shape curves shape cylinder shape disk shape ginstance shape implicit shape nurbs shape plane shape points shape polymesh shape procedural shape sphere shape
All these nodes have a name parameter (for example, polymesh.name) that is a unique string identifier in the Arnold universe. So when you use the Object color mode, you get a unique color for each node.
If you’re using Softimage, note that SItoA puts the frame number into polymesh.name, so you’ll get a different color on each frame if you use the Object color mode. For example, on frame 5 a polymesh might be named “Dog_Mesh.SItoA.5000”, and on frame 6, “Dog_Mesh.SItoA.6000”.
In this render region comparison, you have frame 5 on the left, and frame 6 on the right:
In a case like this, you would use the Object ID color mode instead. SItoA automatically assigns unique IDs to the shape nodes (MtoA doesn’t).