There are already fungible coins on the market, which are much more fungible than zcash.  In fact, zcash is one of the least fungible coins around, because of this option to "go anonymous" OR NOT.   As such, there are 2 kinds of zcash: those that are "pure" and never got dirty on "getting anonymous" (mined in the clear, and bitcoin-style traceable).  These have the same fungibility aspects as bitcoin (that is, they can be colored).
And then there are the anonymous notes of zcash, which are AUTOMATICALLY stamped "anonymous" and maybe "suspicious", but which, amongst themselves, are perfectly fungible.

Now, THAT's a mess.  More than 90% of zcash is "bitcoin style" just as colorable, and 10% is "anonymous" fungible.

If you compare that to something like monero, or shadowcash or the like, the coins are much, much more fungible, because much much more alike, and essentially impossible to colour without ending up colouring the whole chain.

Making anonymity an option is the biggest blunder zcash ever made.  It is a no-go.