The UK did not fully adopt harmonized colors in one go.
The first thing to switch were flexible cords, presumably driven by the desire to harmonize standards for electrical equipment. This was before my time, but a thread over on the IET forum suggests the harmonized colours in flexes were allowed from 1963 and required from 1971.
Then came green/yellow earths in fixed wiring. Again this was before my time, but the thread on the IET forum suggests that green/yellow earths in fixed wiring were allowed from 1970 and required from 1977.
That is how things remained for a long time. My understanding is that the IEE (now the IET) were reluctant to adopt the harmonized phase and neutral conductors because of issues relating to three phase. Firstly the possibility for confusion where old and new colors met and secondly the issue that at the time the harmonized colors did not distinguish between L2 and L3 introducing the possibility for phase rotation errors.
Then in 2004 (with an allowance to use up existing cables until 2006), the UK finally brought in full harmonized colours for fixed wiring. As part of this the harmonized colors themselves were changed, introducing grey for L3 (in older harmonized cables both L2 and L3 were black).
Further complicating matters is that 3 core SWA is most commonly produced in 3 phase color codes, but it is also possible to buy it in single phase+neutral+earth colors. I think this was the case even pre 2004 but my memory may be failing me.
So 3 core SWA exists with at least three and possiblly four different color codes.
Red,Blue,Yellow: old 3 phase colors
Red,Black,Green/Yellow: pre 2004 single phase colors (not 100% sure on this one, I think it existed but my memory may be failing me and I can't find any photos right now)
Brown,Black,Green: harmonized single phase colors
Brown,Blue,Green/yellow: harmonized single phase colors
When cable coded with three phase colors is used for single phase plus neutral and earth, the cores used for neutral and earth should be reidentified with tape or sleeving, but in reality this does not always happen.
With the pre-2004 cables the norm, when using a 3 core SWA with three phase colors for single phase was to use the blue conductor for neutral and the yellow conductor for earth. With the post 2004 cables, the electrical industry associations pushed grey neutral and black earth, but many DIYers are likely to have used black neutral and grey earth. Great care should be taken that conductors are correctly identified.
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