Hello David
This is a question I too have often asked and always seem to get mixed responses on. In my experience colour really does make a difference, but trying to work out on a particular day which will work well; thats anybodies guess?
The method I have adopted is based on simply using a colour or pattern that would represent the bait fish of the area you fish. For example when I am GT fishing at one of my "outside, good visability" places and yellowfin are present then I would throw something that looks like a yellow fin - or at least somthing that has good silver flash on its sides, just like a tunu does - which I think is the draw card for the GTs, if you look at the belly of your favourite poppers and stick baits, as a GT would from his attack position, I think you will see that they all look very similar, but those that roll like stick baits with the flash on the side or even some of the better poppers with a colour like the prey, for me do seem to get more GT attention than just grabbing out a random weight and sized popper or one that is a flat colour. I believe it's the old wounded fish / easy feed thing.
In saying that, if I am playing with the smaller fellows in our local canals which have less visablility then I find the dark or black work better as I think the profile will trigger the attack response more so than flash or a colour....Everybody will have different experiences with different colours but I have spent quite some time experimenting when the bite is hot and this is what I have found, I hope it helps.
Peter