It's time for me to take a guess, everyone to the poolroom please.

(It's just speculation though)

I think the murderer was

Mr. White

The pieces we have to the claim is:

1: Mr Green was a Lizard.
2: The poolroom could have a pool instead of pool tables.
3: Nearly everyone has a perfect albi.
4: The butler was at the crime scene by the door.

Now comes the interesting part:

There's not a single mention of Mr. White, and we only know one single thing about him: He's a chameleon (as his daughter is also one).
He has a daughter so he can't be the pair of father-son in the lounge (the other daughter being the late Mr. Green's)
Since he's a chameleon he can turn invisible, so it's highly probable he's the missing 5th person on the ballroom, watching everything unfold.

But... how? Why there?

Lizards can easily die due to temperature.
The weather was nice in the morning but turned rainy in the evening. I don't know about London's temperature (even the U.K. is foreign to me) but it should be quite cold, though not enough for a lizard to die.
...unless he was soaked to the core...
So Mr. White called for Mr. Green for a chat in the poolroom, but alas Mr. Green saw no sight of Mr. White.
Mr. Green called the butler to ask for Mr. White, something the latter didn't expect, so he had to act quickly. Just when the butler came in he saw how Mr. Green "accidentally fell" into the pool, dying quickly.
The butler couldn't help him. Perhaps it was too shocking? Maybe he couldn't swim? Could Mr. White be blocking him? Or perchance he actually rescued him and moved him to the ballroom but it was still futile? After all, Mr. Green was soaked and he needed to be warmed quickly.

Going forward a bit, to the final scene

The maid overheard some wet footsteps, which could be Ms. Red coming in from the rain, or the butler trying to save Mr. Green (hopping into the pool and fetching him out)
Three of the Ms. were dancing, the butler either came in to try to save Mr. Green or to relay the awful news to Ms Green.
Ms. Red, the one outside, is perfectly undestandable: Bears can go hunt in the rain after all, Mr. Red couldn't though, since he's handicapped.

But wait, why was Mr. Red hurting so much? Well...

Lizards smell very foul when dead, and bears are known to have very acute noses. That could be reason enough, or maybe he was very hungry as well, and that's why he asked (sent) his wife to go hunt, since he couldn't do so personally. The foul smell is bad enough, on a hungry stomach it's just a nightmare.

And so, in the ballroom at the end, we have:

Ms. Green
Ms. Blue
Ms. White
The butler
Mr. White, who is still so invisible he wasn't mentioned in the story at all

It's time for me to take a guess, everyone to the poolroom please.

(It's just speculation though)

I think the murderer was

Mr. White

The pieces we have to the claim is:

1: Mr Green was a Lizard.
2: The poolroom could have a pool instead of pool tables.
3: Nearly everyone has a perfect albi.
4: The butler was at the crime scene by the door.

Now comes the interesting part:

- There's not a single mention of Mr. White, and we only know one single thing about him: He's a chameleon (as his daughter is also one).
- He has a daughter so he can't be the pair of father-son in the lounge (the other daughter being the late Mr. Green's)
- Since Mr. White is a chameleon he can turn invisible, so it's highly probable he's the missing 5th person on the ballroom, watching everything unfold.

But... how? Why there?

Lizards can easily die due to temperature.
The weather was nice in the morning but turned rainy in the evening. I don't know about London's temperature (even the U.K. is foreign to me) but it should be quite cold, though not enough for a lizard to die.
...unless he was soaked to the core...
So Mr. White called for Mr. Green for a chat in the poolroom, but alas Mr. Green saw no sight of Mr. White.
Mr. Green called the butler to ask for Mr. White, something the latter didn't expect, so he had to act quickly. Just when the butler came in he saw how Mr. Green "accidentally fell" into the pool, dying quickly.
The butler couldn't help him. Perhaps it was too shocking? Maybe he couldn't swim? Could Mr. White be blocking him? Or perchance he actually rescued him and moved him to the ballroom but it was still futile? After all, Mr. Green was soaked and he needed to be warmed quickly.

Going forward a bit, to the final scene

The maid overheard some wet footsteps, which could be Ms. Red coming in from the rain, or the butler trying to save Mr. Green (hopping into the pool and fetching him out)
Three of the Ms. were dancing, the butler either came in to try to save Mr. Green or to relay the awful news to Ms Green.
Ms. Red, the one outside, is perfectly undestandable: Bears can go hunt in the rain after all, Mr. Red couldn't though, since he's handicapped.

But wait, why was Mr. Red hurting so much? Well...

Lizards smell very foul when dead, and bears are known to have very acute noses. That could be reason enough, or maybe he was very hungry as well, and that's why he asked (sent) his wife to go hunt, since he couldn't do so personally. The foul smell is bad enough, on a hungry stomach it's just a nightmare.

And so, in the ballroom at the end, we have:

Ms. Green
Ms. Blue
Ms. White
The butler
Mr. White, who is still so invisible he wasn't mentioned in the story at all