Ok, that is a really wild guess, but let's see how it goes.

Could it be

The "party" is actually a pool/billiard game?

My reasoning

There is 12 guests, 2 employees and 1 mysterious host. That's make the 15 balls in full rack
  We all assumes that husbands, wives and children have all the same color, but the ones that are explicid confirmed are:

 - Mr and Ms Green/Lizard - Those would be balls 6 and 14
 - Mr and Ms Red/Bear - Those would be balls 3 and 11
 - A girl who like purple - either ball 4 or 12
 - Ms Blue and a boy in blue (notice there is no mention of husband/dad's color) - balls 2 and 10
 - Ms White and a girl in white - Oops. There aren't two white balls in the game, right?
  Well, according to Wikipedia, in English billiards (the "party" is in London), there used to be two White balls, but these days they replace one for a yellow one, so they could be balls 1 and 9? Maybe? That would be what the line about the girl being a chameleon is referencing
  So, the remaining balls are the other people, that don't have a color explecity stated. The host would be ball 8. The maid and the butler would be one of the remain pairs and so on.

About the house

Diamond is a brand of tables
  The six rooms (Poolroom, Kitchen*, Dining*, Library, Lounge and Hall) would be the holes, wraping around the "ballroom", which is the cloth part.
  3 balls felt in the kitchen's hole 4 balls felt in the Lounge's hole The butler is at the door because he's almost falling into one of the holes

Now here is the tricky part:

The 5th "person" in the ballroom is the cue ball.
  The location of the two Reds and the host doesn't matter, because they where pocked by the losing player (let it be P1)
  P2 had already pocketed the 7 previous balls. When he pocked one the Green Ball into the poolroom, he won the game. That's why everyone else is still alive, but Mr Green was killed when he got pocked

Ok, that is a really wild guess, but let's see how it goes.

Could it be

The "party" is actually a pool/billiard game?

My reasoning

There is 12 guests, 2 employees and 1 mysterious host. That makes the 15 balls in full rack.
We all assumes that husbands, wives and children have all the same color, but the ones that are explicid confirmed are: 
- Mr and Ms Green/Lizard - Those would be balls 6 and 14 
- Mr and Ms Red/Bear - Those would be balls 3 and 11 
- A girl who like purple - either ball 4 or 12 
- Ms Blue and a boy in blue (notice there is no mention of husband/dad's color) - balls 2 and 10 
- Ms White and a girl in white - Oops. There aren't two white balls in the game, right? 
Well, according to Wikipedia, in English billiards (the "party" is in London), there used to be two White balls, but these days they replace one for a yellow one, so they could be balls 1 and 9? Maybe? That would be what the line about the girl being a chameleon is referencing.
So, the remaining balls are the other people, that don't have a color explecity stated. The host would be ball 8. The maid and the butler would be one of the remaining pairs and so on. 

About the house

Diamond is a brand of tables.
The six rooms (Poolroom, Kitchen*, Dining*, Library, Lounge and Hall) would be the holes, wraping around the "ballroom", which is the cloth part. 
3 balls felt in the kitchen's hole 
4 balls felt in the Lounge's hole 
The butler is at the door because he's almost falling into one of the holes 

Now here is the tricky part:

The 5th "person" in the ballroom is the cue ball. 
The location of the two Reds and the host doesn't matter, because they where pocked by the losing player (let it be P1).
P2 had already pocketed the 7 previous balls. When he pocked one the Green Ball into the poolroom, he won the game. That's why everyone else is still alive, but Mr Green was killed when he got pocked.