Nice thread. You can also easily add a border by using the "add new layer style" which is the fx button. Just hit stroke, change it to inside and you can also change the size. A lot easier than using a brush imo.
Another thing is you can set the background color and then change canvas size by a however big you want your border and it will appear to have a border. You can do it multiple times to make a multi colored border. ie black and yellow like you used.