Yes, marbles change color often. It shouldn't stop changing color, it's part of who they are. They should get duller when they get older, though. Orange dalmatians don't change colors. Only marbles change colors and Bettas who are taken care of more nicely.
They will change some or a lot.
I love that about marbles! It's like having a fish every week
Here's a pic of the boy in my avatar at different stages... he died in June but has still remains my favorite.
This article from bettyspelndids with pics might help...
why does this happen?
The marble pattern exists because of genetic transposable elements, otherwise known as a "Jumping Gene". These are certain genes which are capable of moving from one location to another on an organism's chromosomes. Sometimes they will insert in places where they interfere with gene expression. This results in a cell's (and its daughter cells') inability to perform certain tasks associated with that gene. Because the 'jumping gene's' residence at a particular place in the chromosome is only temporary, the inability of the gene interfering to express itself is only temporary. This is what causes the change of pattern in a marble betta, and sometimes the complete and utter change of color.
If a jumping gene is present and it inserts itself into a gene responsible for producing a colored pigment, it stops the production of colored pigment, and all the progeny cells will be unable to produce color. This results in a colorless patch appearing; looking like a white or cellophane patch. The reverse can happen too! If the cells are unable to produce color because the transposable genetic element is present and then the element leaves, the progeny of this cell will be able to produce color again (reverts) and a colored patch will appear.
Confused yet? Perhaps this will help to clarify things. The below series of photos was taken by fellow breeder Ilse, of the Netherlands, and depicts a solid colored turquoise Crowntail betta that obviously carries the marble gene, although it is not evident in the first photo! Photos used with permission.
Here we see a pretty standard solid colored turquoise betta.
In this photo the fish is clearly getting a bit older (note the longer fins), and is still a nice little turquoise. But what's that?? THERE! On his head! This is about the time those hobbyists who had have limited experience with marble bettas think, "Oh no, is my betta sick?"
In another week or so the pale discoloration continues to grow. In spite of its appearance the betta still acts very healthy. This is usually about the time the amateur gets nervous and might be tempted to start medicating their fish for everything under the sun -- DON'T! Unless your betta is acting sick and lethargic or you can clearly diagnose what the trouble is, put the medication away. You could easily kill a perfectly healthy betta by unnecessarily medicating it. And you never know if next week your "discolored" betta might turn into...
Taaa-DAAAA! Even the amateur would be hard-put at this point to deny their betta has a marble gene. You can clearly see the way the "discoloration" has lightened into a flesh-colored patch and is beginning to spread over the torso. The jumping gene is also affecting certain areas of the finnage by creating white and colorless streaks.
This boy ain't done yet! The Jumping Gene continues to eliminate the colored cells until the betta has a flesh colored body. It continues to affect the fins by reducing the expression of the turquoise and replacing it with white or clear. The red "wash" that was always present in this fish from the beginning now stands out more noticably, creating a very nice, contrasty, colorful tricolor marble!
A little later you can see how ruthless the Jumping Gene is in its quest to liberate this poor fish from its turquoise. The blue color literally appears to be melting away. The only thing unaffected was the red pigmentation scattered over the body and in the fins. And now, let's take one last look at our boy:
Solid turquoise to Cambodian Butterfly?! That's why we love the marble gene. You just never know what you're gonna get. And get...and get...and get....