Dan,
Well I know the best course of action would have been to crack the whistle right away. I think I could reply to a lot of posts on this board with, "well, I would have done it exactly right the first time instead." Not sure that helps anyone though and it really does come off as holier-than-thou.
There had been several substitutions during this dead ball so both coaches were just kind of standing there, watching their respective offense and defense prepare for the throw-in and not really paying much attention to my partner. As soon as I hit my whistle, both coaches realized what had happened and so did my red-faced partner. I walked toward the table and both coaches had figured it out by then and both were laughing. The P.A. announcer made a quick announcement "throw-in error, should be white ball" for the clued-out fans and we went on without incident.
I just don't get Chuck's take that we would have been screwed if the white coach had known the rule.
"Coach, I recognized it before the throw-in was complete but I hesitated due to my inherent insecurity."

End of story.
How screwed would we have been had I let the play go on and then told the white coach, "Well sure coach, I recognized it before the pass was thrown, but by rule I'm not allowed to fix it once the ball is touched inbounds?" I think at that point, I start looking for the escort out of the gym.
Z