I think you are quite right about fish in the canals appearing to ignore certain lures and colours. The problem is made worse when you have caught a few fish in the past on a particular soft bait colour. You know “that one” works because you’ve been successful with it in the past and so you stubbornly refuse to try something else!
The rainbow in this picture took a Dragon-V-Lures Bandit Lunatic gold coloured soft bait, with black spots, a green back, and flashes of red (pictured above). I’d been fishing a black dressed jig head that had been successful previously, but with no luck. A couple of casts with the gold soft bait and I hooked this rainbow straight away! I guess if it isn’t working it could be time to try something different.