Post-Crisis Jason Todd had black hair, which was not dyed, as is seen in his flashback origin story back in the 80s, where Bats discovers him trying to boot his tires. This has actually been posted to this comm. This red hair business is a retcon by Morrison as a callback to the Bronze Age/ Pre Crisis .
Four Boy Wonders with black hair and nothing but height and costumes to differentiate them from each other is a bit weird and dull. Yes, it's always a bit freaky that all Bruce's Robins have been beautiful black-haired, blue eyed boys who could pass for his biological sons, but that still be seen as sheer circumstantial co-incidence, coupled with that Bruce may see younger versions of himself in these boys. Having one of them actually change his natural hair color, on the other hand, looks like some sort of weird fetish no matter which way you look at it. Also, it MAKES NO FUCKING SENSE.
Adding a bit of the pre-Crisis Jason to make one of the Robin's immediately standout from the herd isn't a bad idea.
Jason was the one Robin who was actually killed, haunted the whole Batfamily for years and then came back as the black sheep of the Batfamily who was the only Bat-child to challenge Bruce's two golden rules: no guns, no killing. In the face of that, how is changing his hair color any meaningful way to stand out? No to mention that red-haired children have had a long history of being depicted negatively in myth and folklore.
Jason was killed off because nobody liked him. Jason was killed off because the writer didn't like the entire concept of Robin and actively worked to make him as distasteful to the other fans so that he could kill him off. Dick dodged that bullet by reason of being in the Teen Titans and Tim avoided that fate by a combination of becoming less Batman's partner and more his protege, being relegated to his own title and Chuck Dixon.
If that has changed it's because of the way he's been written now. The majority of Jason fans love Winnick's reimagining of him in Under The Hood. That is very different from the way he's being written now, from a psychotic child-killer in BftC to Morrison's DildoHeadMan here, who continues to be an irrational psycho.