In keeping with the color theme of the truck I would use a silver outlined, black letter. The litter silver will make the F-150 pop just like that blacked out FORD logo. At least it looks like a black FORD logo in the picture. Also you have silver wheels. Good luck.
What color is a tattoo? Don't they call those tramp stamps? Seriously, Just kidding ... I would probably go with a hue darker or lighter than the paint color.