Hello DBT,
All your questions are answered in this document:-
http://www.rgsonline.co.uk/docushare/dsweb/Get/Rail-41171/rt2483.pdf
Scroll down to Page 9 section C and you will see that it is a railway regulation and that various shades of yellow are permitted.
The area shall be a minimum forward facing continuous area of yellow surface of 1 m² with a minimum dimension of 0.6 m when viewed, at a large distance, head on from in front of the vehicle. It shall be permissible to divide the yellow surface by elements of an end gangway if necessary. Each section of a divided yellow surface shall have a substantially uninterrupted yellow area not less than 0.4 m², with a minimum dimension of 0.6 m.
Is that clear?
