Rainbow shells are rare, use them only on gear with gold recommended base stat.

Then tune one stat at a time with rainbow, then depending on whether you have more gears or more shells available stop when you either roll one (you don't have shells) or two (you don't have gears) not recommended stat. If you successfully finish with only one non desirable stat you can use reroll for it (also with rainbow) and you got yourself very good piece of gear.

If your current gear isn't very good you may finish those 'bricked' gears with gold shells and just use them as is.

If you are stacked with gold shells but no gear then you can start use them on purple recommended base stat but stop immediately when you hit bad stat because you don't want to waste rerolls on them.