Another question we answer often here at dotnumberstore.com is "Does the US DOT require me to have a specific color for my lettering?".
The US DOT wants your lettering to contrast so that it is visible against the background of what you are installing it on. That being said, if you are purchasing vinyl stickers to more permanently adhere your DOT numbers to your truck, you will want to choose a color that is light if your truck is painted a dark color, or dark if your truck is painted a light color. If you are opting for removable magnetic signs for your US DOT lettering, you can choose any color background, and a contrasting lettering color, as long as what ever the color is behind the numbers contrasts with the number color, you will be compliant.
In other words, don't choose black US DOT decals for a black truck, or white DOT stickers for a white truck! Lastly, the DOT does not care if the letters are permanent or temporary, as they do understand that companies often rent vehicles for transportation of goods, meaning yes, it is okay to use either US DOT Stickers or magnetic signs on your company vehicle or semi.