Welcome to the forum!
Betta colors will get more vibrant and may even change a bit with proper care. My guys have all changed at least a bit since I bought them. Usually it is an enhancement of the colors due to increased health.
Marbles are a different story. These fish have a particular gene which leads them to drastically change color throughout their lifetime. Sometimes you would swear it is a different fish! I have a marble boy who I bought as a pure white betta with about 6 greenish blue scales and blue lips. His body is now completely dark blue/green, he had some orange on his head a few weeks ago but it is black now, and his fins are white with a black streak on the anal fin and blue markings throughout the caudal and dorsal fins.
Now for some advice: as a new betta owner, I highly suggest that you take a look around the forums and read up about proper betta care (which is much different than what most pet store employees will tell you). Here are a few tips to get you started:
1) Bettas are tropical fish. As such, they absolutely need a heater (adjustable heaters are best). Keep the water around 80 F for optimal health.
2) Your betta requires a tank of at least 2.5 gallons. Smaller tanks are difficult to heat, do not provide adequate space for mental enrichment, and require daily water changes to keep clean, which is stressful on both fish and owner.
3) One of the best things you can buy is an ammonia test kit. You always want the readings to be 0 ppm (ammonia is poisonous to fish!).
4) You will want to learn about the aquarium nitrogen cycle, as this helps you understand how/when to change the water in your tank. Remember, good water quality is the most important thing for keeping your fish healthy. Here is a link I highly suggest you read: http://www.tropicalfishkeeping.com/b...-cycle-353074/
20 gallon long: 3 adult Neolamprologus similis + about 11 fry of various ages; low light planted tank
20 gallon long:2 freshwater dwarf puffers (Puff Puff and Poofer); medium-light planted tank
10 gallon: 1 male betta named Wormy; low light planted tank
10 gallon: 1 male betta named Dante; low light planted tank
2, 5.5 gallon tanks that are currently empty (I see more fish on the horizon