Lutri, Pauper Otter’s companion restriction cares about the color of a card’s expansion symbol, not its rarity. If a card has no expansion symbol printed on it, it doesn’t violate Lutri’s companion restriction even if its rarity isn’t common.

Expansion symbols on playtest background cards don’t count, even if they’re partially visible.

Your companion begins the game outside the game. In tournament play, this means your sideboard. In non-tournament play, it’s simply a card you own that’s not in your starting deck.

Before shuffling your deck to become your library, you may reveal one card from outside the game to be your companion if your starting deck meets the requirements of the companion ability. You can’t reveal more than one. It remains revealed outside the game as the game begins.

The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard, nor to any cards added to your deck after the game began (such as with the conjure mechanic).

If more than one player wishes to reveal a companion, the starting player does so first, and players proceed in turn order. Once a player has chosen not to reveal a companion, that player can’t change their mind.

If you reveal a companion outside the game, for as long as it remains there, you may pay {3} any time you could cast a sorcery (that is, you have priority during your main phase and the stack is empty). Once you do, you put it into your hand and it behaves like any other card you’ve brought into the game. For example, if it’s discarded, countered, or destroyed, put it into your graveyard.

Paying {3} to put your companion into your hand is a special action. It doesn’t use the stack and players can’t respond to it. Once you take this action, you may cast that card if it’s legal to do so before any other player can take actions.

You may have one companion in the Commander variant. Your deck, including your commander, must meet its companion requirement. Your companion is not one of your 100 cards.

The companion’s other abilities apply only if the creature is on the battlefield. They have no effect while the companion is outside the game.

The companion ability has no effect if the card isn’t revealed as your companion at the start of the game, and it creates no restrictions on putting a card with a companion ability into your starting deck. For example, Lutri, Pauper Otter may be in your starting deck even if your starting deck contains cards with silver, gold, orange, and/or purple expansion symbols.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.