Underpainting is first painting numbers in a color that doesn't distract from the color of the translucent dice. This prevents the color used in the numbers from visually "bleeding" from the other facets.
The d12 is underpainted in silver, the d8 isn't.
It also allows this set to have copper numbers with an ice blue die.
Knowing when to do it (and when not to) is an art form.