At Sunset, Allison Arieff put together a short but important list on how to avoid common color mistakes - we chose our top two:
1. Looking for colors that exactly match favorite objects
Allsion gives an example of choosing the exact same color as a "favorite pillow". This seems like an easy choice, but if you want that accent piece to still be a focal point, going for a "similar but softer" version of the color is more successful. It allows the original favorite color to still "pop".
2. Opting for white or cream just because it’s safe
It's great to go with a neutral and it does often make the choice easier, but it is still wise to bring home some samples. Taking the time to see how a few different tones of your chosen neutral react to the light in your particular room will really help with the success of your choice. Allison recommends using "the sample-size colors or oversize color chips that paint stores offer" to ensure ending up with a color you'll be happy with in the long run.
Check out the rest of the list and more good paint picking advice at Sunset | 4 Color Mistakes (and How to Avoid Them)