I must admit this experiment went a bit crazy. I tried to paint the purple-blue part as fast as possible, with loads of water and big puddles (expecting to achieve a smooth gradient). It was a total and absolute failure for the class, but I loved so much to achieve this sort of marbled texture. I most definitely want to recreate it for other projects. I want streaky! not straight lines, but organic wavy shapes like these:
However, I'm still salty that I couldn't achieve a perfectly smooth wash. Once I get more cotton watercolor paper (this was Canson XL again), I'll test different ways to achieve it.