I've been playing around with the different options suggested in this question.
Does anyone have any insight in the pattern brush option? I've tried using the blend tool for three different boxes and colours as suggested. This is what happens using the 'smooth colour' option (I need to use white as the central colour for some reasons, a transparent box doesn't seem to work?):
Then I apply the smooth colour option on the blend tool between each box:
...with no options for corners or edges etc.