The short answer is that it took a while for liveries to be standardised; in fact it took a while for 'the Powers that be' at the Railway Executive to even decide what the standard livery was to be.
Less than a month after nationalisation a 'beauty contest' was organised to decide upon the future BR livery. Held at Kensington Olympia on 30 January, a selection of ex-LMS 4-6-0 locomotives were paraded in GWR green, LNER green, Southern malachite green and LNWR black (which apparently stole the show.) Additionally there was a SR electric locomotive in light blue.
Coaching stock included a variation of GWR chocolate and cream, a red one which was some way off being true Midland crimson lake, and an EMU in Southern green. Despite all the effort, it was largely a PR exercise and no firm decisions were made other than to apply the GWR green to express passenger locos, and a further exhibition at Marylebone was arranged with yet more liveries on show.
After this, it was announced that liveries were to be standardised. Royal blue with black and white lining was to be used for heavy duty express passenger locos, green with black and orange lining was to be applied on all other passenger locos and all the rest in black.
However, this decision didn't last long, and in the summer of 1948 14 trains in experimental liveries appeared in service on selected mainline and cross-country routes. Again this was a PR exercise, with the public being invited to send in their comments to the Executive. The trail liveries were:
- blue with lining of red, cream and grey for the most powerful express passenger locos
- green with lining of red, cream and grey for other express pasenger locos
- black with lining of red, grey and cream for mixed traffic locos
- unlined black for freight locos
- plum and spilt milk lined with bands of yellow-maroon-yellow seperated by lines of spilt milk for main-line corridor trains
- chocolate and cream lined with black and golden yellow, also for main-line corridor coaches
- maroon lined with golden yellow-black-golden yellow for local suburban coaching stock
- green for EMUs
After the experimental liveries had been given several months' trial, the lining on some of the black locos was quietly dropped, and the lining of green engines was changed to orange-black-orange. The plum and spilt milk and chocolate and cream for coaching stock was also abandoned, although it was many years before the liveries themselves disappeared from rolling stock. It was at this time that the famous blood and custard livery became standard. Non-corridor coaches and non-passenger coaching stock were painted red overall.
The answer to the question about how long it took for liveries to be applied to coaching stock is no simpler. Although pre-nationalisation liveries disappeared very quickly, it took a long time for the BR standard liveries to appear on all rolling stock, not helped by constant changes of heart at Marylebone Road (and its heirs and successors.) Although blood and custard was subsequently dropped, and Southern green was only supposed to be carried by multiple units, both liveries managed to cling on. Trains formed of a mixture of blood and custard, all-over maroon and rail blue/grey were not uncommon, even in the 70s!