When two heroes in a row are wearing similar colors, the same colors, or particular combinations of colors, they can "Create Team" to create a combo attack. This is the third option under "Attack" and "Magic" in your menu.
When your teammate joins the fight, together they'll have the option to "Combo Attack." If the colors are similar (ie, light green and dark green, or yellow and orange, pink and red, or purple and blue) they'll simply attack a bit stronger than they would have otherwise.
Here's a video of a "similar color" team in action:
If their colors are the same, they'll be able to do an extra strong version of their standard magic attack instead. Note that not all of the standard magic attacks are damaging towards enemies - some of them are simply beneficial towards your team.
In certain situations, you'll need to send in a combo of your team to handle a particular obstacle. These obstacles include:
- Ghost with Shadowlight Shield - Black + White
- Ice or Frozen room - Red + Red
- Poison or Gas - Yellow + Yellow
- Room of Fire - Blue + Blue
There are also a couple of "single specific ally" obstacles:
- Dark room - One white shirt ally
- White/blinding light room - One dark shirt ally
I used a couple of resources to compile this information which you might find useful. This guide, and this IGN Wiki guide were among the most beneficial sources of information.
These locatons unlock in future playthroughs. Unfortunately when you finished the game (which I am assuming you did) and you selected the secret mission you made yourself ineligible on this playthrough to unlock the doors. See this IGN thread. It is generally suggested to play through the game multiple times unitl you have visited all of the paths and THEN tackle the secret quests.
So basically if you want these paths to unlock you can't take on the secret quests you need to just select a new playthrough.
Best Answer
According to Shadowmathfreak's FAQ, "The mummy will scare off the first hero in line unless the Mii’s color is the same as the mummy." So to stop being ambushed by the mummy, your first hero that the mummy encounters needs to be the same colour as the mummy in question. In the case of a bright/dark/etc room with a mummy, you need to clear the room's obstruction first and the hero right after will need to be the same colour as the mummy to avoid being scared away.