I dyed my hair Neon Red by using “Splat” hair color in their “Raspberry” color. The only place I found it being sold was at Walgreens, (Walmart didn’t even sell it).

When using splat dye be VERY careful as it will stain ANYTHING it touches. If you accidently get some on the floor, try using rubbing alcohol or denatured alcohol to get it off. Also, it doesn’t come off easily from skin, so don’t rub it into the scalp when you are dying your hair. I used a Qtip dipped in one of those solutions and scrubbed really hard. Also be prepared to rinse for a long time to get the excess dye out.

All that aside, I loved my red hair, although between dyings it looked rather… orangish. I should have dyed it more though.
The first time I didn’t bleach my hair so it was a dark red. However by the 4th time I dyed it red, I was bleaching all my hair. It’s a pain in the ass though when you try to dye your hair a natural color to look more “professional” to get a job. I have since dyed it brown 3 times and you can still see tints of red.