If we want to support colored coins (which I think we should) then it should IMHO be part of the core protocol (because you can expect it to become a very crucial part of the IOTA ecosystem). There are just so many use cases …
I am not sure it would even be possible just with IXIs, but you are anyway right we don’t need full UTXO for colored coins. We could also have sth like “UTXO-light” which only keeps track of the balances of differently colored coins separately and still accumulates the balances in each group.
This might be a viable alternative if we conclude that UTXO doesn’t scale well enough. I am however more interested in the conflict detection aspects.