A light red background in the column "Restrictions" in the waiting room means that you are not suitable and therefore are not allowed to join that particular game offer. There is one exception, which is explained below.
 
The restrictions applied for a game offer are shown in an abbreviated form in the "Restrictions" column. The used abbreviations are given below as examples in green color.
 
A waiting game offer is suitable when a player matches the requested game restrictions on:

• Handicap-type: For Conventional or Proper handicap you need a valid rating for calculations.
   
  
• Rating range: the players rating must be within the requested rating range, e.g. "25k-2d".
   
  
• Number of rated finished games: the player must have at least the given number of rated finished games, e.g. "RG[2]"
   
  
• Accept same opponent: the player may not be allowed to join the same game offer more than once. The different options and abbreviations are explained mode in How can I prevent that the same user joins my game offer more than once?
  Examples: "SOT[1]" or "SO[1x]" or "SO[>7d]".
   
  
• Max. number of opponents started games: the number of the players started games must not exceed the server-specific limits; a violated restriction is indicated by "MXG".
   
  
• Hide waiting room games: the player can use the contacts feature to hide game offers from certain users in the waiting room. Game offers that match this "restriction" are marked by "[H]".
   
  If the only matching restriction violation is Hide the waiting room games of a particular user, the game offer can still be joined, because that option only influences WHAT games are shown in the default search for suitable games. The player can still see and join the games via the "All waiting games".
   
  Do not mix the contact system category "Hide waiting room games" with the system category of "Protect waiting room games", which protects your game offers from joining. "Hidden" game offers can still be viewed searching for All waiting games, while "protected" game offers can not be found at all.
  

If any of the above restrictions is not met, the red background indicates the game offer as non suitable. In that case, the game offer can not be joined (except if the only restriction is "[H]", see "Hide waiting room games" above).