I’m not seeing where there’s a problem here.
SO dyes hair, spills dye on seat.
Sits on seat, stains butt.
Now SO’s butt matches hair. What’s not to like?
And OP sits on seat, stains butt.
Now OP and SO have matching his-and-hers butts.
This is all very fashionable.
You can’t get it out. It’s forever.
Odd, that something which only lasts a month or so on your hair and then must be refreshed, is absolutely indelible on other surfaces.
Replace the girl so that you no longer have to worry about the seat.
Replace the toilet with a Chinese one and forget about the seat. Your other problems will eclipse that entirely.
I’ve found that baby wipes (the generic store’s own brand as opposed to brand name ones with X Y and Z in them) are great for removing hair dye from almost anything. I’ve never tried it on a toilet seat though.
YMMV
I tried peroxide (I put a gauze pad over the spot and soaked it, then left it overnight) to no effect. Maybe it will eventually fade with cleaning.
If a Magic Eraser hasn’t worked (and it should, being a glossy surface), mix up a thick paste of white vinegar and baking soda. Cover the stain and leave it for 15 minutes before wiping it off. You may have to do it a couple of times, but it should get the stain out.
Other than that, I’d paint over it with white nail polish.
Does that actually work? IIRC, C[sub]2[/sub]H[sub]4[/sub]O[sub]2[/sub] + NaHCO[sub]3[/sub] => NaC[sub]2[/sub]H[sub]3[/sub]O[sub]2[/sub] + H[sub]2[/sub]O + CO[sub]2[/sub] – a salt (sodium acetate), water, and carbon dioxide gas.
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The wooden ones from Walmart are not particularly durable, especially since the metal fitting is prone to corrode and break.
My dad had a cushy seat in the '70s. I didn’t like it. The inside seam seemed like a germ trap. Also, the outside seam burst after a few years.
I don’t know about the Wal-Mart seats, but the Bemis one I got from Home Depot has nylon hardware. Also, they have ‘twist locks’ so you can remove the seat for cleaning without unbolting it.
Ew, I hate those squishy ones. They soak up moisture like a sponge and ass sweat and urine are not things you want soaking into your toilet seat. 
We had a squishy seat once and I don’t recommend it. It cracked open in about a year.
I’ve used the paste to remove a small hair dye stain from a melamine cupboard door (blonde hair dye - I don’t know if that makes any difference) though it was attended to straight away; I don’t know whether the length of time before it’s applied is a deciding factor.
I may have erred in calling it ‘baking soda’ - it’s actually bicarb soda here - which I thought was referred to as ‘baking soda’ in the US. It’s very possible that in that sense I’m a bit stoopit: further supported by the fact that the chemical formula above means absolutely nothing to me 
spray it (with paint)
It’s an abrasive, but milder than anything being sold as one. Also, if you notice, the end result is basic (in the pH sense, not the simplicity sense). You start with an acidic mixture, end up with a basic one. So, physically it’s a mild abrasive; chemically it’s one mad strip of pH paper.
There is something you seem to be applying too little of, based on your description of how you used the peroxide: elbow grease. You’re not supposed to just leave whatever you’re using on top of the seat, you’re supposed to rub the seat with it.
It’s not hair colour – it’s decomposition. Not toilet seat dyed – toilet seat died.
Aha! I’ll pick up a tube of elbow grease when I got to Rite-Aid! 
I want to be cautious about using an abrasive. I’m sure it would make quick work of the stain, but I don’t want to over-do it. I don’t mind a bit of scrubbing, but I want to choose the right chemicals so that I’m not using too much time (a commodity that is in short supply nowadays).
mom get off this board!!!
If the chemical is right, you can tell within seconds; if it’s really, really wrong (doesn’t remove the stain but damages the substrate), leaving it in place will do the damage you’re trying to avoid while leaving the stain intact.
Elbow grease doesn’t necessarily involve using an abrasive. If I applied polish remover to a cotton ball and left it on top of my painted nail, all I’d get eventually is some fucked-up nail decoration, polish transferred from the nail to the skin, cotton fibers stuck to the polish that remained on the nail… putting a bit of remover on the cotton ball and “scrubbing” the polish off is what works.
