Red Summer Pareo and Coronal Summer Halter! We can dye the coeurl version of the exact same set, so why not the patternless one too. We also don't have pretty much any plain bikini tops without patterns etc that are dyeable, and a lot of the basic but dyeable ones dye horribly. For example the Summer's Flame top, which dyes into a weird faded color, and same with the Endless Summer top with the color fade at the bottom and the straps turning grey when you dye it any color.
And piggybacking off of that, stop putting weird grey shading on gear. This has been a thing I've noticed a lot in Endwalker gear. Almost everything has that muggy grey shading that makes the clothes look dull especially in light colors. Examples I can think of off the top of my head are the Exarchic and Rinascita sets (I guess you could argue it's just part of the aesthetic but in my opinion it still doesn't look very nice with light dyes), the Valentione Emissary's Culottes which have an annoying grey shadow at the top of the legs, and the weird grey buttcrack/crotch you get from the Isle Vacationer's Tanga that older similar bikini bottoms or shorts do not have. We already have a super limited dye system as it is and everything looking muddy as hell unless you're using dark dyes is making it even worse. I hope adding a more advanced dye system is in the plans for the future.