I've seen more than a few unpainted clear receivers at games. There were several unpainted Krakens at Border Wars last year. Since higher quality G&G and ICS clear / smoked guns have become available in Canada, I've seen quite a few of those at games too. And they were clear, not painted.
That being said, most of the owners of said guns intended to paint them eventually just because it looks better. But honestly, I don't really see how a clear receiver could give someone away on the field any more than a black receiver.