Hey presto, colour! If you do it whilst the Beeb is switched on (N.B. Not recommended unless you are as reckless as I am) then the colour appears before your very eyes!
The only slight side effect is that the picture is not quite as sharp with the addition of colour, if you compare it to the earlier monochrome picture. The colours are not as vivid as when using the RGB output, and the overall brightness of the picture is lower than other output methods, but it's a vast improvement on the old way of pointing a camera at the screen!
So why didn't Acorn make colour Composite Video a standard feature of the BBC Master? Surely it would only have added about a farthing onto the production costs!