In Commander, can you build a deck around a face card of a modal double-faced card as your commander?

Under Commander rules (and CR 903.3a, updated with Zendikar Rising rules), a modal double-faced card may be designated as your commander if the front face is a legendary creature card, a planeswalker with the 'can be your commander' text, or otherwise meets the eligibility criteria. Only the front face's characteristics matter for eligibility — the back face is irrelevant to this check.

CR 712.4a establishes that in zones other than the battlefield and the stack, a double-faced card has only the characteristics of its front face. This means your commander's color identity, mana cost, and creature type are determined by the front face alone (though CR 903.4 does also include mana symbols on the back face when calculating color identity).

One important nuance: color identity uses both faces (CR 903.4). So if your front face is a mono-green legendary creature but the back face has a blue mana symbol, your deck's color identity includes blue, and you may include blue cards.

Concrete example: Valki, God of Lies is a legendary creature on its front face, making it a legal commander. Its back face, Tibalt, Cosmic Impostor, has red and black mana symbols, so the deck's color identity is black and red (from both faces combined). You could cast it from the command zone as Valki for {1}{B}, or cast the back face Tibalt for its 5BR cost if you have a way to do so legally.

Note that you may only cast the back face from the command zone if a spell or ability allows you to cast the back face specifically — normally from the command zone you cast the front face, per CR 903.13.