This is a little picky, but the original text of some old cards have activated abilities that don't use colons. Alpha Black Lotus, for example. Your correct, though, that currently activated abilities will have colons 100% of the time.
Grim Reaper 2:symb::symb:
Creature - Horror
When Grim Reaper comes into play, put a demise counter on each creature.
At the end of each player's turn, that player sacrifices a creature with a demise counter on it.
4/4
Creature - Horror
When Grim Reaper comes into play, put a demise counter on each creature.
At the end of each player's turn, that player sacrifices a creature with a demise counter on it.
4/4
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