I have found the relevant box at the back of the loft and leaved through a pile of nicely browning papers. So far I have not found the specific reference I thought was there so you will have to go on memory for now.
The blue/yellow/brown/green colours were established either late in LMS days or early BR and were enshrined, for control panels, in the BR Standard Signalling Principles. Four colours were the minimum needed to avoid two similar coloured track circuits being adjacent. It was certainly LM Region practice to keep blue/yellow for one line and green/brown for the adjacent line but this rule had sometimes to be broken in complex areas to avoid adjacent track circuits of the same colour. It was not mandated so far as I can remember, and not in the standard, as to which colour was applied to the up line and which the down but there was probably a custom and practice default. The red track circuit, as I mentioned on Sunday was reserved for the track circuit that controlled the block instruments. I remember that being written somewhere but haven't found it yet. Other colours, usually only plain black for running lines not track circuited and background colour with outlines for sidings.
BR Standard Signalling Principle Number 25 of March 1965, paragraph 3, included the following
"Each track section is to be distinctively coloured using green, blue, yellow
"Push buttons will have coloured escutcheons and switch knobs will be themselves coloured in accordance with the following":-
|Function||Push button or switch|
|Auto button||Light blue|
|Level crossing release||Brown|
|Crank Handle release||Brown|
Note that this was generally carried through from the traditional lever colours but with some subtle differences.