hold on to your mouse,here it comes : my first tutorial ! :cy:
hokay now ... grab the polygonal lasso tool and,in a new layer, select an area aproximately like that one (but without curved corners) ... hold down shift for 45degree increasements ... something like this : http;//www.ipb.as.ro/tut1.gif
ok now ... go to Select>Modify>Smooth and smoothen it by 6-8 px or smth like that .
Fill the new selection with the desired colour .