Can a Colorless commander use cards with the {C} symbol on them?
Quoting from the Commander rules:


  Cards in a deck may not have any colours in their color identity which are not shared with the commander of the deck. (The identity of each card in the deck must be a subset of the Commander's)


Colorless isn't a color, and since Deceiver of Form doesn't have any additional color symbols in its rules text, you are allowed to play it.

Examples of cards you're not allowed to play in a deck with a colorless color identity Commander:


Culling Drone (black color identity because of its mana cost, regardless of Devoid)
Boros Signet (red and white color identity because of mana symbols in the rules text)
Elbrus, the Binding Blade (black color identity because of its color indicator on the back face)


You also won't be able to use any lands with a basic land type, such as a Swamp, due to this rule:


  Lands whose type includes swamp, island, plains, forest and/or mountain (basic lands, shocklands, dual lands, Shadowmoor special-basics, etc) also have the corresponding color identity. (...)