[rootlinux]

Are you talking about the border that also displays the contact name?
If so that is a manila file.
You will need a manila editor to extract the png to edit.
Then you will need to replace that png on the manila file with your edited png.
At work and I forget the exact manila file name.
Very easy to mod it though.

Depending on whos rom you are running and what theme is on it, the default could be any color.
My roms have silver,black, white, red ect.