It can be hot but it depends on your facial features, skin colour, style and haircut. It can also have an opposite effect. You know, if you're a tomboy with short bright red hair it usually sends a "lesbian message".
I love girls with bright colored hair. I did date this 1 girl that had bright hair and whatever dye she used seemed to run a lot, it be on her, her clothes. I don't know how that works but she would look like a mess sometimes. Other than that I loved it.
My opinion is similar to @foxloversi 's. I think bright red hair can very hot, in my opinion, any girl whose hair is almost shoulders length will be hotter if she dye her hair bright red, therefore, there are girls who can be attractive for me just for dying her hair bright red.
Yesterday I saw some guys whose girlfriend wore dyed red hair and it made me think they are lucky for it.
The only disadvantage for a girl to dye her hair red is she doesn't look as elegant as with her natural hair color.