use hair chalk/ col hair gels /non permanent shampoo colors. its best if you want short change and your hair back. I also have loong hair and last holidays it was all purple using shampoo colors (⌒▽⌒) it just washes out with hair washing.
Ofc its possible certain products can make your hair couple days blue and then vanish... I don't know which kind of products you can use in your country but you can ask cosmetic shops
If you're worried about your hair not turning back to it's natural color, maybe dye only the ends? I've seen many people do it and they still look nice
Idk how well the shampoos and stuff other people are recommending are going to show through brown hair, I've never used it.
But if you're looking for more permanent, it's going to have to be bleached out first (depending how dark your hair is). That'll make the color actually pop nicely. You can always dye your hair back to a natural brown, a good stylist should be able to match your natural color nicely so as your hair grows out, it'll all look natural when it comes back. I've dyed my hair like every color, currently just have purple peekaboos. I have very dark hair, so bleaching is a must.
Okkkk
So I really want to dye my hair in blue but I never dyed my hair before. I have brown hair that touches my butt. The main problem is that I want to change my hair but for only a couple of month bc I like how my hair is now. I want to be able to return to my natural hair and idk if it's possible