[...] I like the idea of painting the threads green, which would make a change from the usual red. Do you have any evidence to support the green colour?
There is no actual evidence for
any particular colour for the fringes. No coloured representation of a standard of this form has survived.
The notion that the fringes
might (note the continual emphasis on
might) represent a grass bundle is based on:
1. The independent assertion of two Greco-Roman writers that grass and leaves were Roman standards in archaic times.
Plutarch, The life of Romulus, 8.6:
"He led a large force, divided into units of a hundred men, each unit headed by a man who carried a handful of grass and leaves tied around a pole."
Ovid, Fasti, III:
"They used to carry a handful of grass on a long shaft."
2. The physical appearance of the objects. Many of them look quite convincingly like a bunch of leaves and/or grass. For example, consider the lowermost object on the standard carried by this typical
signifer on Trajan's Column (early 2nd century):
The object immediately above it is a leaf crown. The same basic sculptural technique is used to represent the surface detail of both objects.
3. Roman culture's well known veneration of antiquity and respect for tradition.
Green would seem an appropriate colour for a [presumably] soft fabric object that symbolizes a bunch of grass and/or leaves.
You're totally free to reject this thesis, of course, but I just thought you might have been interested to at least know of the possibility.
One other snippet of information about the colours of standards is the statement of the 1st century Roman encyclopaedist Pliny that
"ilver is preferred for [the metal parts of] our military standards, its brightness being visible over a greater distance." (Pliny, Natural History, 33.19)