the tunnel...

The tunnel is something that's generally used to show they're entering a new world, frame of mind, or something. It should remind you of Spirited Away because he used it in the same way, although he originally had something far more elaborate planned but decided it would be too long.

Ponyo saying "I don't like this place" can probably have any number of meanings, but I think the simplest is the best. She was scared because she was entering an unknown place and could probably tell that something was going to happen. She could probably sense that her magic was getting weak and she didn't want to go back to being a fish and being away from Sosuke.

One thing Miyazaki has mastered in his films is capturing the essence of childhood and really being able to accurately express how they feel and react. This is just another case of that, most children at that age would probably have a pretty similar reaction.

The color of illusion is perfect blue

In the DVD menu credits, it lists this chapter as 'Sosuke's test'. I'm still not clear on what his test was exactly and how he was supposed to pass it. It seems all he did was enter the tunnel, reassure Ponyo and raced to get her back in water as she began reverting back into a fish.

It seems to me that he'd have done that regardless of any tunnel or test considering that he did it all throughout the film. So I still don't see how this was any sort of test or challenge.

Anyone else care to elaborate?

Wow! I wasn't the only one that found the tunnel to be similar to Spirited Away.

That said, I was largely disappointed afterwards. It was such a nice, clean and carefree ending. I was expecting a lot more from this film.