I can actually give a small update on this topic.
If everything goes well we will have coloured edges 
We’re just getting approved the first designs of Monzo debit cards and we’ve decided to invest a bit more to replace the white layers that sandwich the contactless circuit (the dark thin layer you can see if you look at the edges of your Monzo card) for hot coral plastic.
We don’t know yet how the actual result will look but we hope it’ll make the card feel more like a solid object made of hot coral material 
