developer and DEMI permanent

i never used developer before.

i tried artic fox purple af which is semi permanent i have dark brown hair so it didn’t really show on my roots. if i purchase a different brand of purple demi and mix with 20 volume developer over my already dark purple hair will it turn out crazy ? i don’t want it to be super bright but also don’t want it so dark that it only shows up on my ends that were highlighted. i have no experience with demi or developer so let me know guys thank youuuu

edit: i would prefer not to bleach my hair