Hi,
I'm lucky enough that the songrapher had the sex right with my first two monkeys, this time around I had a scan at 17 weeks, 20 weeks and then a 4d at 24 weeks, all clearly showed the penis. I have heard of wrong predictions through family and friends in most cases they were expecting a girl which turned out to be a boy.
If the scan isn't very clear and the fact that it was clearly changed through the scan I'd wouldn't rely on it being right. I know that some ultrasound 3d and 4d places offer a sex determination scan, the one I went to was about $120 and included photos ect. The 3d scan clearly shows the sex and if your really determined to know it might be a good investment. Otherwise I'd just stick to neutral colours and before you know it your little bundle will be with you and you'll clearly know what sex bubs is!
Best of luck,
Snoopea