Some interesting insight on this during the February open office during the Q&A:
Excuse my dodgy transcription, but this is the question and the answer:
Question: One of the goals of the [recent tube advertising] campaign was to increase brand awareness. Was it a conscious decision not to really focus on the multicoloured M? …
Erm, that’s a - err - an interesting question… Erm … I’m loathe to answer this because we’ll see what happens with the multicoloured M …
Basically, erm, to be totally transparent I started here around 5-6 months ago and was completely against the multi-coloured M and would… I wanted to get rid of it.
BUT it has a helluvalot of brand equity and does a lot for us, because it is… the second you download the app that is the first thing you see and first thing you get to know with the brand past the card. The funny thing is with the card we don’t have that and haven’t had that for a long time, however long ago it was - a year or two ago - it still had the outline M on the back of the card. I don’t know if anyone still has that version? But since we’ve got rid of it and we just run with our logotype. Again, it’s a bit of a watch this space situation - it’s something that we consistently near to bear in the mind - we either need to use it in an obvious way in our upcoming campaign or think of a way around it for the whole of the brand. But we’ll see, it’s all - up here right now, and hasn’t been explored properly yet.
This is really interesting, and it seems like there might be changes in the future. I’d love an open office / blog / Q&A on here dedicated to this type of thing - especially before changes are finalised. I wonder if @simonb could pull any strings?
As an aside, this reminds me a bit of Mozilla’s recent rebrand, where everything was published - draft treatments included - publicly and was a fascinating process to follow. It was also clear about its principles, namely that they weren’t crowdsourcing the answer, there was no voting and no one was being asked to design for free). I wonder if Monzo would be up for a similar form of public brand evolution? It certainly fits the brand (pun intended).
But back to Monzo: I’d like to see an evolution of the 
logo to be transparent and white, sitting on a hot coral background - a little like used to be on the back of the cards. I think this marries two of the most valuable parts of the Monzo brand - and I think there’s a lot of utility having a short ‘M’ version of the logo - see Facebook and Google instantly recognisable by their initial. I wonder if Monzo could do something similar.