The post effective was the black on black
I also printed the 1 square posters with Typo Twelve in large ltters across the 12 posters. I tried foiling the Typo however the lines were too thin to really be effective. Also the black on yellow was not as noticeable.
I then made the decision to somehow incorporate black into the posters. The only way I could include laser printing (to achieve the foiling - will only work on Laser ink not inkjet) was to use it like a frame to attach the yellow squares to. With more time I am sure I could have found a more appropriate solution ie digitally ink jet print then use a screen to foil the black.