If I dye my hair, how easy will it be to go back to brunette?

For the colors you want, yes, you'll probably have to lighten them, especially for the red at least. The purple could be done without bleach, depending on how dark your hair is exactly, but if you want dark purple you shouldn't lighten your hair too much at all. And yeah it'll be weird to go back to brown, but what I did before was choose the color closest to my hair color and it turned out pretty nice. The dyed hair will look lighter than the natural hair because it's been bleached, but you could also try a shade that's just a tiny bit darker than your natural hair color to even out the brightness, but in my experience the box color I used transitioned well with my roots, it wasn't perfect, but it didn't look bad at all.