For ki manipulation in that way, it would be considered magic and would need the tattoo. The only way ki wouldn't need a tattoo to be used would be if it's more of a passive effect, or if it's a something like a strengthening buff.

As for the element, it can be either. Usually it will take on the element of the ability, but in the case of ki, you could use any element and just adjust the abilities to match. So an icy ki blast is doable!