InstancedMesh: Honor instanceColor in copy().

src/objects/InstancedMesh.js

@@ -33,6 +33,9 @@ InstancedMesh.prototype = Object.assign( Object.create( Mesh.prototype ), {
 		Mesh.prototype.copy.call( this, source );
 
 		this.instanceMatrix.copy( source.instanceMatrix );
+
+		if ( source.instanceColor !== null ) this.instanceColor = source.instanceColor.clone();
+
 		this.count = source.count;
 
 		return this;