Please note,
Serissa,
Sageretia t.
Southern Chinese elm
Murraya p.
Podocarpus
and there is a good chance - Fukien tea out of China is also Sub tropical.
A grower in New Jersey winters his Serissas out side and there are images on-line of Sageretia leafless and with snow on the branches.
As far down as Guangzhou, is a zone 9 - chance of frost and snow.[ Fukien tea zone ]
This is most probably why folk growing the above indoors, get them dying for no reason.
They can live down here [ West Indies / Caribbean ], because from late December to possibly middle February it is cool enough for them to rest.
You can see when everything stops growing down here.
The same way the West Indian or Barbados Cherry [ Malpighia glabra ] naturally ranges into South Texas and also lives happily down here. As do a great many trees and shrubs, they can still rest.
Now the equator, who knows?
Probably on mountains of over 3000 to 4000 feet in height [ plateau zones ] and higher.
So yes Serissa can be offered for white flowers.
I know the Ilex y. can also have white stars and it grows well down here - chuckle.
Don't mean to be rude - apologies in advance.
Good Day.
Anthony