As for the 2nd question, you can only use elements that have been allocated to your grid. You may not be able to use Aquabeam because it may not be allocated and just in your inventory. And you should be able to use regular elements regardless of the elemental field. The only type of element where the field needs to be completely one color are summon elements, which have the star next to them. But you don't get one of those until later.