I've seen people with the same item, but with different colors.
How does that work? The items in Mabinogi's store change colors every day at 12am (game time). Check the store often to see if they are selling your desired color that day. You can also create your own item with your desired color. When you are manufacturing the item, you can choose a finishing material or use the Dye Ampoule, which is an advanced item, to add your finishing touches.
I saw a bunch of people sitting around a campfire.
What's the deal with campfires?
A campfire provides an intimate environment for you and your friends to share stories and chat. Also, when you sit near a campfire, the warmth helps the recovery of your HP and wounds. You can also make food at the campfire and share it with your friends. When you share food, the effects of the food are felt by everyone!

When I connect to the game, on the top it says, 'Today is (day of the week)' along with an effect... what is this? How does that work?

Weekday effect is a phenomenon that affects the mabinogi gold  world. Depending on the day, different effects occur in the game. You can check what effects happen on which day by looking at the message when you connect to the game.
Mon: Alban Eiler Affects Production
Tues: Baltane Affects Quests & Dungeons
Wed: Alban Heruin Affects Combat & Magic
Thurs: Lughna Sadh Affects Enchant & Gathering
Fri: Alban Elved Affects Shop Purchase & Bank Service Fee
Sat: Samhain Affects Character's age & Skills Training
Sun: Imbolic Affects Combat

The item window turned red
Weapons, shields, armor, and items you can equip all have adurability level. The durability of an item decreases when equipped. When the durability reaches to 0, the item is no longer useable. When this happens, go to the appropriate shop and repair the item.
Are there any important differences between playing during the day or at night? (Game time)
During the day, you can get part-time jobs throughout the town. At night, your mana recovers faster and you can transport between towns using the Moon Gate. Only in Mabinogi, you can experience the business of working during the day, and the tranquility of fishing under the stars during night.