Can I use spells with dual mana symbols against Iona, Shield of Emeria?
You can't cast Unmake.

Those are called hybrid mana symbols.

105.2. [...] An object is the color or colors of the mana symbols in its mana cost, regardless of the color of its frame. An object’s color or colors may also be defined by a color indicator or a characteristic-defining ability. See rule 202.2.
107.4e [...] A hybrid mana symbol is all of its component colors.

So Unmake is white and black, so Unmake is white, so you can't it.