Spectrum adds a wide variety of items and resources, and some of them are handy to have on-hand at all times. Of course, I could use a backpack mod (like I'm currently doing) to carry all of that stuff around, but wouldn't it be so much better if we had a cooler solution coming from Spectrum itself?

It could be a very interesting "modular" kind of storage item, where you could get opportunities to improve its functionalities as you advance in the mod, such as:

• First of all, something that likely would come by default with the belt, which would be a pigment pouch (patent pending :P), which as the name implies, can contain all of the different pigments there is. Whether or not it would show missing pigments that can't be obtained yet is up to you, it would personally not bother me either way.
• Additional individual pouches for the different gems (and their crushed variants), with the belt allowing you to add more of those pouches as you discover said new gems.
• The ability to attach one (or perhaps even multiple!!!) bottomless bundles to it, so they don't clutter your inventory as much (and makes it a good alternative to, say, dank nulls).
• The ability to perhaps enchant it (or its individual pouches) to make the stack size bigger per slot, so you could have two, three, four stacks of one dye per slot instead of just one.
• Something like an attachable holster for a tool, so you could quickly holster it and take it out, once again for inventory management reasons.
• An "ingredient" pouch, for stuff such as Shimmerstone, Quitoxic Powder, Vegetals, Neoliths, Stardust, Glow Ink Sacs, etc. It would allow your Spectrum trinkets and tools to draw their required ingredients from there, when they need them, so you don't have to carry that around in your inventory to be able to use them (on its own, with just Spectrum, it's not /so/ bad, but in modpacks, having to carry a bunch of stuff around just makes me go "I'd rather not use it" because of how much of an issue inventory clutter is).