I have no idea if it would have stained the gravel or not. Since the driveway is not completely yours, I suppose that anything you or your neighbors do to it affects all of you.
While I don't think any harm would have been done in the long run, I think you were wise not to have poured it out there because you already had a reaction from a "third party" who showed you what at least a few others might think of it. Better safe than sorry. Better to pour it into the storm drain or down a sink where people expect liquids to go.