Doubling Cube’s ability is a mana ability.

The “type” of mana is its color, or lack thereof (if the mana is colorless).

Any restrictions on the mana in your mana pool aren’t copied. For example, if you have with no restrictions on it in your mana pool and that can be used only to cast artifact spells, you’ll end up with , that can be used only to cast artifact spells, and that can be used for anything.