I need to make a lot of star cookies for my daughter's school, and need to have them blue or silver. I'm considering either using fondant to cover them or royal icing, then either painting or airbrushing them. Which would you recommend as being easiest? 

I use fondant.

Make sure to have all the fondant cut out before you even put the cookies in to bake.  Then as soon as they come out, lay the fondant on top and the heat will "laminate" the fondant onto the cookie.  Work fast!

Thanks leah_s! I've done that before, just wasn't sure with the painting if it'd be easier another way. Would you then paint or airbrush? I'm wondering about messy edges. Or can I color the fondant pieces before I put them on the cookies? Wasn't sure if it would still work if the fondant is more dried out. Thanks so much.

I think I would colour the fondant blue and grey. Roll and  cut out the stars and cover with plastic so they don't dry out. Then use a silver dust and blue dust either mixed with clear vanilla or vodka and paint. For me that would be the easiest way to go but we all have our preferences.

rolled buttercream is especially good on these kind of cookies too and the same way leah said to cut the icing out before you bake but with rb you also put the cut out icing stars in the freezer as you put the cookies in the oven so it handles well to apply them as soon as the cookies come out of the oven-- be sure you get edible silver etc.