What should I do with this hair dye confusion?

You HAVE to bleach your hair first. You cannot put colour over brown hair, You will most likely (depending on what dye you use) get a mushy blueish colour that lasts for a day. Also, you always, always have to obey the MI (manufacturer’s instructions). If the dye says ‘leave on for 30 minutes’ and you leave it on for 40, there is a good chance your hair will snap off and you will have a bald spot. I am a hairdresser, and even though we know everything about colours and bleaching etc, we always follow what it says on the MI. Every dye is different.

Reds/purples will fade much easier that blues.

But yes, you have to bleach it. If you bleach brown hair once, it might go orangy copper, you will have to bleach it twice, you should go to a hair salon.

Its like (for example) imagine black carpet, if you spill red wine on it, the carpet will be wet, but won’t be stained because its so dark. It may be a tiny bit stained, but nobody will notice. But if you have yellowy/whiteish carpet and you spill red wine, you will end up with a reddish orange stain. If you have pure white carpet, the stain will be the exact colour of the wine.

So, the lighter your hair is, the better the colour will hold.

Also make sure you use a colour-save shampoo on it.