I bought my car used, it is a 2006 Scion tC. The original color of the car is silver, as I gleaned from looking at my door jamb. However, I believe the previous owner painted the top half of the car (the hood, half of the mirrors, and from the windows up) a metallic blue color. I would like to repaint a section of the car the blue color, but I'm not sure how I could identify what specific blue it is. Any suggestions?
It is very hard to color match automotive paint properly. Professionals (at a body shop, for instance) would be able to do it, as suggested by Paulster2.
A specialty automotive paint retailer (not a general auto parts store) will be able to match colors, but it is often a bit of a trial-and-error process. If you are able to remove your mirror and bring it into the store, they might have a color matching machine that they can put it in.
Take your make and model to a body shop supplier, they will be able to match the OEM color on their computer or book.