Main toolbar > (Render Setup) > Render Setup dialog > Render Elements panel > Render Elements rollout > Add button > Render Elements dialog > Add Object ID element to the elements list (or highlight an existing Object ID element in the list).
Rendering menu > Render Setup > Render Setup dialog > Render Elements panel > Render Elements rollout > Add button > Render Elements dialog > Add Object ID element to the elements list (or highlight an existing Object ID element in the list).
Render Color Based On
Object Color Renders colors based on object colors.
Object ID (The default.) Renders colors based on Object IDs.