The BBC Computer Colour Modification is an electronic circuit modification to convert a monochrome video signal into a colour one. The modification is for the earlier BBC Model B Series 1, Series 2, and Series 3, as these machines provided a b/w output from the composite video socket.
Back then, the industry considered RGB output, as the only proper method of providing colour video signals, and a composite video with colour would not be good enough. Television technology has improved over the years and you can obtain good quality full colour display from composite signals.
This method is to convert the composite video to colour by adding the colour burst signal to the output video waveform.
This modification is minimal and does not require removing the PCB or even opening the RF Modulator.
I have managed to boil it down to two points of soldering and the addition of a single 56-pF capacitor.
The red lines in the animation above shows the wires. These should be as short as possible and obviously insulated all the way up to the capacitor terminals using wire insulation sleeves.
The first point of solder is R123, which is a 470 Ω resistor. Simply solder to the top of its terminal, which is easily accessible.
The second point of solder is the emitter of Q9, which is a BC549 transistor.
Essentially, what we are doing is joining the emitter of Q9 and the base of Q7 through a 56-pF capacitor. This modification mixes the colour burst signal to provide a composite colour output.