You can use silver, or any other color that is in the palette you loaded.
Or you can use another image to pick the colors from, but it needs to be a .png image.
So you could just download greys palette image from anywhere and use that if you don't want to create new palette.
Read the description and instructions (very small text I know) below the settings.
There seems to be no way to enter manually color codes.
![Image](http://gimpchat.com/files/2783_Colorize-Interactive.jpg)
Greys and black used, quick test.