At least in the case of marinades containing acids (such as vinegar or lemon juice) or certain enzymes, especially from papayas, kiwis, or most commonly (at least in the US), fresh pineapple juice, a certain amount of denaturing of the proteins will occur at the surface of the marinated meat. This will turn it opaque rather than translucent--chicken will look kind of white. This is perfectly normal, and safe.
Depending on the color of the marinade, and the particular cut being marinated, some of the marinade may also penetrate into the meat, changing its color as well.
These are normal effects, and assuming you have otherwise respected good practices (keeping the meat refrigerated during the marination, not holding the food uncooked longer than would be safe without the marinate, and not cross-contaminating and so on), the product should be safe to eat.
In the case of your meat marinated triple the planned time, the outcome will depend on the nature of the particular marinade. If there was active enzymatic action, it could be very mush to the point of unpleasantness. On the other hand, if the marinade was mostly for flavor, rather than highly acidic, it might just have a more intense flavor from the marinade.
I should add for clarity,
when safe above means
when properly cooked...