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.
Do you personally have a preference on what paint you use? I'm assuming it's acrylic? Have you found that some brands work better than others?
Normally, I just use the paints I use with my eldest to paint Warhammer minis.
For underpainting, however, I prefer the Vallejo Air metallic, as they're super thin and coat like a dream.