Can I make custom space marines or do I have to pick one of the Main Space Marine Factions?

I'd rather paint my marines however I feel and not be tied down by the rules for the different factions such as Blood Angels or Space Wolves. Can't I paint them however I like?Can I make custom space marines or do I have to pick one of the Main Space Marine Factions?
You may paint them in any way you want i have my own custom painted army and they even have there own custom rules( i.e they can use any codex and have extra hit points) i call them the doomsday marines.Can I make custom space marines or do I have to pick one of the Main Space Marine Factions?
You can make up your own army including a back story and paint scheme but what you CAN'T do is just make up rules for them and give them different stats. And you certainly can't just use any codex you feel like using. If they're Space Marines, use the Marines codex.

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