You buy all the red color variant keys from the character with the yellow key on his head. You need enough money to buy it though.
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Thanks! I can't believe i didn't realise that lol. Do you maybe also know how to finish the 5 stage quest with those letters? For the final hint it tells me to 'let those which are circled flow' or something, and i thought it might have something to do with the circles of the inventory and the sinks throughout the last town, but it doesn't do anything