Adobe Illustrator is one of the strongest tools used in vector based designs, so many of the times you have to play with paths and lines in Illustrator. Paths are basically are made of points or anchor pints where the shapes are formed of boundaries.
Since paths or lines with anchor points are hard to work with specially changing the color or applying other effects. So we have to convert these paths to shapes to fill them easily.
Convert Stroke To Shape in Illustrator :
Here in the image I have created a circle and stroke it to 14px with no fill color. The should not be a fill color for the circle to get the required output here. Now you can clearly see anchor points on top,left,bottom and right. To apply color and other manipulations on it we need it to made as a shape.
After applying outline stroke you will see the circle path into one shape where you can changes colors directly with color picker or in any other way.