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?
Right, that's correct. But I think it should be implemented in a way that doesn't require those to work. For example, in my 'NFT implementation' idea, I could sell you an image's rights by directly sending you the image and proving ownership using any block explorer or even just Bitcoin Core. Or posting the image somewhere / even hosting it myself. It should really be super independent on where the image is stored;
a hash and signature should suffice for ownership proof.
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.
Does it work like that with current 'OpenSea' NFTs and BBGC's 'Bitcoin NFTs' as well? Because I agree that what you describe is how an NFT should work. Independence from any specific centralised (or not) site / service / marketplace. I should be able to advertise and sell NFTs even on this forum in my opinion.