A face-down permanent is just a 2/2 creature. Period. It has no abilities, no type other than "Creature" (in particular it's not an artifact), no subtype, no abilities, no color, no mana cost, not even a name. Whatever is printed on the other side of the card is completely irrelevant, as long as the permanent is face down.
What I'm saying is that your face-down Colossus of Akros isn't a Colossus of Akros at all, as far as the game is concerned. It's just a nameless vanilla 2/2. And as JonTheMon said, a nameless vanilla 2/2 isn't an artifact and doesn't trigger Scrap Trawler's ability.
The rule that says so, albeit in more complex language, is this one:
707.2. Face-down spells and face-down permanents have no characteristics other than those listed by the ability or rules that allowed the spell or permanent to be face down. Any listed characteristics are the copiable values of that object's characteristics. (See rule 613, "Interaction of Continuous Effects," and rule 706, "Copying Objects.")
707.2a. If a face-up permanent is turned face down by a spell or ability, it becomes a 2/2 face-down creature with no text, no name, no subtypes, and no mana cost. A permanent that enters the battlefield face down that wasn't cast as a face-down spell also has these characteristics. These values are the copiable values of that object's characteristics.
Of course, all that changes when you flip the card face up. Then it switches to being whatever is printed on the front of the card (in your case, a Colossus of Akros). Same when it leaves the battlefield.