HAIR PROBLEM?!?! RED HAIR?

I am Portuguese Eurasian, 14, female. I live in Asia and have had extremely brown, almost jet black hair. Recently as I grew older, my hair has been turning brown, and now it's a mixture of brown, black, red with a purplish toy? It gets redder every year! This changes in hair color i also being experienced by my elder sister. My teachers have been asking me to dye my hair black as they don't believe it's natural! I've only does my hair dark brown once last year and I've dyed it BLACK back but the color has faded since then. Should I dye it black again? Will it work?