We recommend an Aluminum color paint for signals. The very best paint for signals (in my opinion) is Model Master Metalizer Aluminum. Sadly, this paint is no longer available but there are some alternatives that work well too, The Metalizer paints were laquer based so did not need a primer ahead of it. I would recommend using Vallejo Metal Color paints that can be airbrushed after priming the model with a laquer based white primer (they can be brushed on as well but generally are intended to be airbrushed). I have found that Tamiya primer is best for priming all of our kits - it goes on very thin and dries pretty fast.
Vallejo Model Color paints have several different shades of Aluminum and we sell the Duraluminum color on our website. This one is a little bit darker than fresh aluminum and makes the signal look a bit more aged and realistic.
If you would like lighter shades of aluminum you may be able to find the Model Color paints on Amazon..