When one color unintentionally flows or migrates into another colour, or into a part of a painting where it's not wanted, it's called bleeding.
With watercolor painting this is very easily and rapidly done when a new area of wet paint touches an existing area of wet paint. Not only does the wet paint flow together but as the ground (the paper) is also wet the paint also seeps in along the paper fibers. Leaving a thin gap between areas of paint solves the problem; how big this wants to be depends on how wet the paper is and how much paint you put down.
