I had that book! Programming books were generally out of my budget back then, but I saved up for that one, which I recall being full of incredibly clever tricks. I wonder if this is the same David Webb - I guess it probably is: https://en.wikipedia.org/wiki/David_Webb_%28Hong_Kong_activi...

I think switching between colours at 25hz would give you pretty horrible flicker, although maybe we would have forgiven that in the Spectrum era.