How do I get my dyed hair to fade and what does hair developer do?

My hair was almost white blonde, but I couldn’t keep up with the roots anymore so I dyed it a light brown. It faded a lot so I dyed it a medium brown, and it faded to a reddish colour. Then I dyed it a medium brown again and it went black! It has been fading a tiny little bit but is still almost black and it has been at least a month and a half. I want to speed up the fading process so I can grow it out without noticeable roots. Any good tips that won’t damage my hair too much?

Also, when I was blonde I purchased a bottle of 10 volume cream developer and a “lightest blonde/natural ash” demi-permanent colour. What does the developer actually do? (please, in layman’s terms) What would happen if I just put the developer in my hair and then washed it out? Can I somehow use these products now to lighten my hair up a bit? Without turning it orange of course!