I think that you need to be a little more specific about where and when.
Some elements of the BEF in 1940 had white mudguard tips to display their formation signs.
They were often applied on the UK Home Front to aid visibility during the black out.
Later in the war, in NW Europe, I think that they are most commonly seen on Corps of Military Police Provost motorycles...As per the whitened webbing that appeared later, it made them more visible for Traffic Controm etc.
I've not seen anything in the official markings schemes.