@rundocuri- Yes, a dentist should be able to give you a dental veneer and crown that are very close in color to that of your natural teeth. This is a concern that you should talk to your dentist before he or she begins to work on your mouth so you are sure to get the results you want.
Most dentists have color guides that they can hold up to their patients' teeth to get good color matches for veneers and crowns. This helps prevent snow white crowns when the natural teeth are much darker in color. The companies that make these dental materials are also aware of how important it is to dental patients to have matching teeth, so they are careful to check the colors and match them as closely as possible during the manufacturing process.
The people that you are referring to that have mismatched colored teeth possibly have old veneers and crowns. Years ago, these dental devices often looked unnatural and didn't match real teeth. With today's dental technology, it is usually hard to tell the difference.