I'm not familiar with that model but...
There are two things that are not very surprising.
#1 That the food coloring in the sink and toilet, didn't show results. The food coloring would have only showed up in the drain section of your plumbing, which are only gravity drains that aren't under pressure.
#2 That the problem is showing up under higher city water pressure.
I would close each valve individually, to see if the leak goes away. You might be able to pin down and locate where the leak is.
But I agree with Awchief, that the P trap could be the culprit, but it would probably be a slow drip type of leak. A paper towel might be the easiest way to determine that.