Here’s an interesting article I stumbled upon yesterday from 2016. Did this just fizzle out?
Using 2 digit route numbers with colour coding seems like an even worse way of doing things (as suggested in the article). If anything, I’d prefer to see a letter prefix to support any colour coding which makes the purpose of the route clear. E.g. X for express, N for nightlink, L for loop, H for high frequency.
At the very least, moving to such a system would require a significant simplification of the network, which I think is dramatically needed personally.