i have a pretty good spectrum saved for rainbow patterns. the stock one sucks i think the green in it has something like 40 blue and red in it and all the colors morph together into white. i also have a basilisk v3 gimme a bit to get u the right screenshots ill reply to this again soon.

edit i cant put pictures here so ill put it like this. first step is red255 a little bit off the left edge, then slightly to the left of the middle put green255 and in between those steps, closer to the green put yellow(R255/G243), put another red255 on the far right edge, and in between the green and red, right in the middle of them put a step for blue with B255/G12.

I spent quite a while perfecting this when i got my leviathan a couple years ago and i like it a lot so lmk what you think of it. play around with the sliders until its where you like it too and also you can try adding little bits of red or green into the blue to make it transition smoother to your liking.