When opening Shulkerboxes in your inventory, via Quick Shulker, they have the default shulker gui colour, instead of their corresponding color. (using Colorful Containers)

kyrptonaught/quickshulker#59 (comment)

I took a look at the original issue, and yes, OptiGUI v1 works that way (remembers the last container).
As of now, OptiGUI 2 works with entities and block entities (QuickShulker doesn't place a block entity, that's why it doesn't work out of the box), or general blocks (like loom) with fewer features.
However, since OptiGUI 2 is made extensible, I have some ideas how to make these mods work together. The simplest solution would be adding some code to QuickShulker, which I can do later (this code creates filters for general shulker box UI based on the opened shulker box item. Since it's not a vanilla feature, I don't plan to add it to OptiGUI).

OptiGUI will have client-side QuickShulker support (2.2.0-alpha.1 and onwards). No server mod update needed.
Yes, I do have a QuickShulker patch, but it will break with the current codebase (because I refactored it).
This way, OptiGUI can remain client-side only, and doesn't depend on the QuickShulker dev to include a patch, and an OptiGUI update won't break QS if I change it too much.
I didn't do it yet, because I found no better way to do it, than passing data from a mixin to another Which can cause funkyness with high ping or server lag.