Why my little plant start changing color from the root?

From the picture, it definitely looks like a root rot. Try to completely stop watering it for a while and see if you can still save this beautiful plant. Keep in mind, that a cactus prefers dry environment, so next time, water it only when the grow medium you keep it in is dry. Hope this will work for you.

It does look like rot. Stop watering it. Looks like a cactus. They like the soil to dry out almost completely before they get watered again.

It's rotting from too much water by the look of it.

Stopping watering it.