The green button is the same as a call for aid on 15x - it will sound a call tone and allow two-way communication with the conductor.
The red button will sound an alarm in the cab but it can be overridden by the driver to prevent the brakes from applying.
Thanks for the clarification - so the same as the one that's only provided in the toilet on most stock?
That setup makes a lot of sense - most issues just require human intervention. It's to nobody's benefit to waste time stopping the train if someone's having a heart attack, for example - indeed doing so may result in their death as it would delay them getting to somewhere an ambulance could attend, which I guess creates the mentality that overriding it makes sense most of the time rather than it only making sense in some cases e.g. if you're on a viaduct or in a tunnel where stopping could be actively hazardous in the event of e.g. a fire.
Thus on a 195 I guess (I don't know if any staff here can comment based on experience) you'd get far fewer uses of the red handle for things that don't require stopping in the first safe location at least to investigate?
It also makes a lot of sense to have a means of getting staff attention, particularly in a longer train, where the issue is nothing to do with safety but is still serious, e.g. the failure of booked assistance for a wheelchair user, or the failure of the Avanti staff to show up to let a cyclist out on a Pendolino (which I did once pull the red handle at a station for*, because it was the last train of the day and the result of not doing so would have been the cyclist being stranded at Euston with a very long ride back and these days probably a letter asking for a £100 settlement for overtravelling - I got a bit of a snark from the guard but the situation was properly resolved with minimal delay)
* I've pulled it twice. Once was that, the other was for medical assistance on the Tube where there was a panicking kid with his (I assume) Dad who had passed out, on an unrefurbished D78 on which it was just an alarm and emergency brake with no phone-like function. That case was handled appallingly - I pulled it and stood at the open door waving, and it took a good 5 minutes for platform staff to notice me and come over, by which time he'd come round a bit so they helped them both off so they could presumably sit down and get the required medical help. They seemed more interested in making snarky announcements implying that it had been pulled inappropriately.