What is the color of the extra power die that the Doom ability gives the Overlord?

If I have a silve rmonster will th eextra power die be black or silver?

Also if the monster is maxed out with a mixture of different colored power die

( for example a Silver Master Dark Priest ), does doom upgrade a black power die

to a Silver one?

In RtL Doom grants a Power die upgrade. So you could add 1 black die (assuming you have 4 or less) or upgrade a black to a silver, or silver to gold.

Instead of +1 die upgrade, does anyone HR this to +1 die upgrade per campaign level? (to be on par with Parry, etc, that get progressively better).