Hi Denise :o)
I had a private scan at 24 weeks with Jacob, and he very helpfully had his legs akimbo and his bum pressed against one side of my tummy, so his genitals were dangling there for all to see!
I think it does depend on the position of the baby at the time of the scan, they generally tend to be squished up with limbs folded inbetween legs or just in awkward places making it difficult to make out.
I wouldn't paint the nursery a particular boy or girl colour anyway- We chose a bright sunshine yellow for Jake's room, you can always imprint the sex of the baby on the room with the sheets you choose for the cot- the curtains, the changing table colour and of course- all those cards you receive after the birth!
I think it's a nice idea to string them up like bunting around baby's room (well out of their reach obviously).