In my mind, coloured coins based NFTs would be much simpler and much less reliant on such websites. Since the utxo is 'tainted', you could transfer it like any other Bitcoin transaction (e.g. with dust limit amount) even without any NFT explorer or marketplace.
You need these sites to present your work. They must work as escrows where the seller gives the marketplace their piece of work and people start bidding. I haven't ever purchased anything, I've just seen them.
How would colored coins be any different? Wouldn't you need to present them somewhere?
An analogy would be if you had the rights to the Mona Lisa, but couldn't prove or transfer them anymore due to one website shutting down due to interest in art ownership proofs dipping down for a couple years.
Whoever has the image and runs a full node can calculate the hash and therefore, verify the ownership of an NFT/colored coin.