From wildlife guide:

"How To Attract
The 'ready for harvest' Piscis Aquabullae will help bring the Kingfisher to your Garden"

If I recall correctly, they used to prefer certain colours. (I think all colours did attract them, but some more strongly.) So, I think there's been some change at some point.

Does anyone know whether (a) they still do have a preference, it's simply no longer mentioned in the wildlife guide, or (b) they're now equally attracted to all colours?

No, they don't have a preference.

Thanks Tracey. They used to, right? I'm not imagining it am I?

I wish there would be some warning/notice when things change (but perhaps there was and I missed it).

Maybe you noted that it appeared that in your garden, a particular colour seemed to elicit more kingfishers than perhaps another colour?

No, my recollection is that a preference was stated in the wildlife guide. Again, I could be wrong about that. But perhaps others can confirm whether this was the case. (I'm going back some time - I don't know when the change happened, if there was one.)

I have been here seventeen years plus and I don't recall that. But then...my mind is not a steel trap.

I don't think there ever were a preference for color for the Kingfisher.  Maybe you are confusing it with the other animals attracted to the same plant, that require a specific color, like the king penguin, walrus, platypus etc.

Do you still need 10 kingfishers for one to be nesting or is it five?

Violet, every 5th of any bird will build onto a nest and every 10th attracted will nest as long as that bird has built onto the nest you’re using in your garden.