Toxic Barrel
~9:30
He claimed that the reason he could change the color of both emission lights at the same time is because he used alt+d, but it would have worked the same way with shift+d in this case.
I assume that this is the type of issue you were asking to be reported.
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alt d - means linked duplicate, all copies share the same properties (color)
shift d - duplication, the copy will have the same property values. But not related (linked) to the original one.
You use alt- d if the objects are repeated in the same way. Less Blender memory needed because there is only one version to maintain.
Using shift d creates copies, more Blender memory. But the freedom to change properties and mesh.
In some course parts, other options are explained. Like exchanging object mesh with other object mesh. This can be used for the level of details. Like a simple cube wall of 8 vertices. Replace the internal mesh with hundreds of vertices. Not copying and past vertices, but changing the inner data properties of an object.
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You are correct. The âlightâ is an object with emission. Any copy will have the same material. He is altering the material not a light.
Well spotted!
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It looks like âemission lightâ isnât as clear as I thought it would be.
Is there a term for an object that is being used as a light? Emissive surface, maybe. 
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