What's the rule about the yellow?

[ECML43]

I worked out when I first started trainspotting that all british trains have to have a patch of yellow on both driving ends to make them more visible to other trains, but what is the rule about it (if there is one). Because the Grand central 43s have only a small strip of yellow below the cab window, whearas most MUs have the entire cab front covered in yellow. Looking at old photos it seems to me like the rule was invented by BR in the late 60s. But what are the specifications of this 'rule' on yellow cab fronts?

[swisstrains]

Everything you ever wanted to know about the yellow warning panel is contained in section C1 of this document.

http://www.rgsonline.co.uk/Railway_G...%20Iss%201.pdf

[ECML43]

Thanks for that info Swisstrains. You seem to have an answer to nearly every question I post! Though I do much prefer it when the answer is on the post rather than sending me off to some other site to look through pages and pages of stuff. I was quite eager to know that though so thankyou anyway.

[swisstrains]

Originally Posted by ECML43

...........................You seem to have an answer to nearly every question I post! Though I do much prefer it when the answer is on the post rather than sending me off to some other site to look through pages and pages of stuff. .....................

I couldn't agree more but short of copying and pasting the actual document it was far too complicated for me to answer from memory.
You could have avoided having to look through pages and pages of stuff if you had gone straight to section C1 as stated in my post but to save you any further inconvenience I promise not to answer any more of your questions.