Personally as gross as it is I particularly like Citrus Yellow. To me it fits the car, plus it was only available for one year (you in the UK get two years of it), it was only available on manual transmission cars for us, basically the deffinition of an Insight. It deffinitely screams "LOOK AT ME!" plus I think it would be hard to blend in to traffic with.
There is also red which I have, it's nice, but was really all that I could find at the time. It was the choice color at that time, but I have to say I'm partial to Monte Carlo Blue. It makes the car look expensive and adds a touch of class somehow.
Silver, while it gives the image of an alluminum car, and is one of the most popular colors, is a bit too bland for me. Most Insight's were painted this color and like Delta Flyer said it blends in and you might not be seen.
The other option you in the UK have is Black (ok really really dark grey). If it wern't so hot here I would be more jealous of not being able to get it in the US. It kind of blends the whole car together with the rear trim being black as well.