If you have blond/light hair and want darker hair -
Mix a few spoons of coffee powder (depending on how dark you want it) in a bowl of water and heat it. Leave it in your hair for 60-90 minutes and rinse it out. Works well and won’t fade until you wash it next time. (Some people say that if you use a lot of coffee and let it stay in your hair for more time it lasts a couple of washes too)
If you have light hair and want reddish hair -
Mix some cinnamon powder, honey and water and apply it to your hair for at least 60 minutes. (Not extremely effective, but leaves a red tint to the hair)
If you have black hair and want blonde/light hair -
This is quite impossible. People do claim that if you apply a mixture of honey, lemon and olive oil (only a spoon of lemon juice at most) for a few months, then it is possible but frankly I haven’t tried it out and I can’t see how it will work. (Use a hair dye instead of bleach though)
If you have dark brown hair and want light/medium brown hair -
Again, very light hair is either impossible or would take lots of months. But you can make your hair lighter. Mix 1 part water, 2 parts honey and 3-4 parts olive oil (you can add a few drops of lemon juice if your hair isn’t very dry) and apply it to your hair. Leave it over night and wash it off the next morning. Do this at least twice a week and the difference will be quite noticeable in a few weeks.
There are a lot of other ways the internet advises but this is just some stuff my friends and I have tried, so I can say they work (hopefully xD). Then again, some hair types are different than others so if the stuff I thought won’t work works, or stuff I said will work doesn’t, just go for a proper dye instead! (Bonus - all these methods will definitely make your hair a lot healthier so it’s good anyway!)