There is a generally accepted rule about neutralizing interior paint colors of a house when placing it on the market for sale. Of course, as with most all rules, there are exceptions. In furnished homes, paint colors that compliment the furnishings can go a long way in enhancing a home's "showability". Take a look at builder's model homes...there's not a neutral wall to be found in these decorator dream homes. Well executed paint treatments can be used to highlight architectural features, warm up large, cold rooms, or make small rooms seem more spacious. However, poorly executed paint treatments can take a potential buyer's enthusiasm down several notches. It's OK by me if you let your children paint their own rooms, but when the FOR SALE sign goes in the yard, your child's artistic freedom could cost you potential buyers.
Ask your realtor, a homestaging professional, or an interior designer if you have concerns about your wall colors. I have often recommended that homeowners repaint their walls before taking photos and putting the house up for sale.
When is it best to neutralize? For certain when the house is empty. Striking accent walls and faux finishes that look spectacular with your furnishings can look downright crazy once those furnishings are gone. In a furnished home, if the wall color is too bright or too dark that it's distracting, for instance the black walls in your Goth teen's room or the vivid colors of your favorite sports team in the gameroom.
Now when I say "neutral" color, I don't necessarily mean white. Warm earthtones of brown/tan, gray/green or soft gold/yellow are preferrable. Blues and pink/rose hues can be tricky. When in doubt, you can always go with an off-white or eggshell.
On a final note, freshly painted walls always help a house show better whether you're neutralizing the color scheme or adding a fresh coat of the existing color.