I do not find the brush selector of grease pencil described on the manual. (https://www.blender.org/manual/en/interface/grease_pencil/drawing/brushes.html)
The manual is for 2.78 which is not official yet...but can be test driven. Check your version before looking for functions that might not be in previous builds.
Brushes become available once you start drawing, or click on one of the drawing buttons in the Grease Pencil Tab - in the Tool Shelf.
You can adjust the colour on the other side in the Properties.