colored silicon

There is silver silicone, you can get it anywhere
they sell sheet metal supplies or HVAC supplies.We
use it to make sure all conections and pentrations
are sealed, and it makes for a nice looking job.

How about this, they sell a product called gutter seal that is aluminum colored and is used to seal aluminum gutters. It's been a while since I've used it, but I would bet that you could find it is a good home supply store. I believe that it was made by alcoa and as I recall it's pretty tough stuff, would have to be to hold up to weather and UV.

HotRod