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What's the best way to make it switchable between colour and b/w displays via a switch added to the Dragon's case?

I'm not sure, I haven't an oscilloscope to have a look at the signals. Particularly you should check how the output of the LM1889 is mixed with the B/W composite video signal. Perhaps you could use a simple switch to turn the output of IC9 on/off since the color burst is simply added to the composite video signal. I guess that the output of the LM1889 is zero most of the time.

Thanks! That makes two of us not having a scope. I will experiment with different methods based on your information.