Hair colouring question
@zoec there's a 'midnight' series from splat that works alright, but it depends some on how dark your hair already is. Most rainbow colors are deposit only dyes and need a lighter, porous hair strand to show up.
Additionally, some dyes can work with a developer. You may want to look up 'high lift' dyes and similar, but their action will be like bleaching, though it's technically not bleach.
Lastly, you could use colored extensions or a weave, though they can introduce their own complications.