Looks like the 'Lock Fill' modifier is turned on in the Tool panel (available at the bottom of the panel when the Brush/Paint Bucket Tool is selected). Check if that is the culprit.
Also, have a look at http://animation.about.com/od/flashanimationtutorials/ss/gradtoolock.htm for more details on how to use that modifier
ps: please mark this post as answered if this is of help to you.
Hi - could you be confusing the Paint Brush Tool with the Brush Tool? The brush tool doesn't allow for these configurations while the paint brush tool does - that might be why it's faded out.
Hope this helps - good luck!