How to choose a paint color?

I'm repainting a church nursery and I'm not sure what color to do. It's going to be jungle themed. I really like this green color from Walmart called "Green Tomato." It's really bright, but I love it. I've heard before that when picking a paint color you should look at the swatch to see what color you like and then buy the paint that is one shade lighter. Is this true? I really love this "Green Tomato" color so I'm going to have trouble making myself get a different color. I feel like the color you say you want should look the same on the wall as it does on the swatch. Does anyone have any tips? Should I buy the color that looks good on the swatch or one shade lighter?