Kids write on the sidewalks and streets all the time here with chalk and at the Uni I work at, the 'greek' students use the public sidewalks like billboards during their recruiting time (I don't know what it's called. as you can tell, I'm not 'greek' lol).

I would steer clear of using all yellow just for public safety reasons - you know their will be some fools that just have to read the message and if they get distracted it could turn ugly. At least with pink or green or blue, if that does happen, you can point out that it obviously wasn't a public notice since it wasn't yellow.