We don't know if it will be a boy or a girl, so we stuck with a neutral colour. We thought it would be nice to get rid of the pinkish-white that the past owner of the house used all over the place. I also thought a neutral colour would be a good idea, so that if we have another baby of a different sex in the future, we won't have to re-paint. Some might call it laziness, but I'll call it efficiency.
We didn't do too much paint colour research beforehand. We probably should have gotten a bunch of paint chips and taped them up on the wall so that we could see what the colours looked like in the light and at night, but we didn't plan things out that well. (There's that efficiency again.)
We trekked over to Home Depot and looked at some green paint. We settled on a paint colour called "Celery Sprig" (410C-3) by Behr. Painting took us a whole day, since we washed the walls first (you'd be surprised how dirty walls can be without looking like it!). We also had to do some patching. We were not very good at cutting in - we didn't use enough paint the first time around - so the second coat really made a difference and made the colour pop.
Here are our Before pictures. This is the problem with taking pictures indoors at night using just a flash - the colours are all weird. The walls almost look yellow, but they're actually a pinkish-white.
And here are our After pictures. The colour looked a little drab on the paint chip, but we used the fancy little machine at the Home Depot that showed us what a room painted with the colour would look like, and it looked much brighter so we decided to stick with it. It definitely did come out bright and cheery.