Many hairdresser have been inspierd by the colors of the celestial sphere.
You can use a wide range of colours to create that effect but really you want them to blend together. Blue, purple and green is a beautifull combination, you can make contrast if you mix pastel and bold colours. The process is quite time consuming but the results are worth it.
Galactic har trend is more about the choice of colour than the technique, you can make horizontal or vertical sections, all is up to your imagination.
This is one of the wildest hair trends at the moment an one of my favourites.