The platform edge marking is defined in a fairly straightforward manner in RGS GI/RT7016
11.2.3 Marking platform edges
11.2.3.1 Platform edges shall be clearly identified by visible marking and provision of a tactile surface.
So it doesn't seem to have to be white, but I can't think of any I've seen in a different colour. There are no obvious exclusions to this requirement - but the difficulty is pinning down a liability on the TOC or NR to carry out preventive maintainance...
I've seen people repainting the white edge at my local (SWT) stations using a quick drying white paint, while the station was open, I think they were SWT employees.