Question

Does Protean give Kecleon STAB on anything immediately or does it take effect after the damage has been dealt?

Comments

I've still yet to find confirmation on whether it is only activated by damaging moves or all moves. Spikes would be an important move to test.

Answer 1

If Kecleon would be, for example, using Flamethrower, before to attack, it would turn to a Fire type Pokemon and get STAB.

Entry: Changes the Pokémon's type to the type of the move it's using.
In-Depth Effect: Before the Pokémon uses a move, it becomes a pure Pokémon of that type.

Here is the source. Yay ! I know now how to "change the name of a link" ! It's by Hyperlinking !
Hope this help :)