Sometimes it can indicate shedding. Shedding skin will often turn lighter color like gray or white. Then eventually peel off and bright new skin will appear underneath. Sometimes this happens all at once, and sometimes (especially as they get older) it happens in patches - even down to a single scale or spike at a time sometimes. It's important not to interfere with or pick at sheds. Providing them with something relatively rough like wood or a piece of carpet etc to rub up against can be helpful to them when shedding.
Another reason for them to change color is to either deflect (lighten) heat/light or to absorb (darken) heat/light. This happens a lot when they are basking or moving between areas of different temperature. They are very good at making the most of light and heat and will move around & change color to adjust their temps as needed.
Changes in color can also indicate mood. This is usually noticed most along their beard. A very dark
black beard may indicate that they are irritated or grumpy or not feeling well (sometimes they are grumpy in the morning right when they wake up, just like us). If they have a bright beard with white colors and often natural orange or yellow coloring, it tends to indicate that they are happy or content. This is one of the many visual indicators that you'll wind up using to determine their mood and personality over the years.