The overall rule for Mono-Deck is very straightforward: Your entire deck, including Sideboard, can only contain cards with a single color (or are Colorless). But, of course, the devil is always in the details. So let's talk some specifics:
- Colorless cards (including ALL Lands and Artifacts) CAN be played freely in any deck, as these cards do NOT have a color. While this isn't true for all Magic cards, it is true for all of the Commons and Uncommons in the current Standard set. Note that this means that Lands that produce multiple colors of mana, such as Cinder Barrens, ARE still considered colorless.
- Permanents that are multi-colored are NOT legal, as they contain more than one color. This not only includes cards like Ahn-Crop Champion but also split-cards like Appeal // Authority. See CR 708 in the official rules, which clearly states that split-cards are the colors of both parts of the card prior to being cast.
- An activated ability including a particular color has no effect on the color of that permanent. For example, Aegis Automaton is a legal card EVEN IF you're not playing White, Avid Reclaimer is legal IF AND ONLY IF you are playing Green, and BOTH Desert of the Fervent and Hashep Oasis ARE legal in any deck.