https://modrinth.com/resourcepack/unique-dark
https://www.curseforge.com/minecraft/texture-packs/unique-dark

This seems to be very similar to Colourful Containers, but it doesn't work with OptiGUI.

It's a vanilla resource pack (the free version) and works for me. Can you explain what doesn't work?

I missed that I had to pay for shulker GUI version. I'm sorry.