How to Make Each Container GUI Match Its Block in Recolourful Containers
The whole point of this pack is that every container gets its own color theme, but you only see that payoff if the pack is actually the one being applied. If your menus still look grey and uniform, the pack is either disabled or being overridden by another resource pack sitting above it.
Confirm the pack is active and on top
Open Options, then Resource Packs. Look at the Selected list on the right. Recolourful Containers GUI + HUD needs to appear there. If it is sitting in the Available list on the left, click it to move it over. If you run other UI or HUD packs, drag Recolourful Containers above them in the Selected column, because the topmost pack wins for any texture both packs touch.
Check per-container results
Once applied, open a few different blocks and compare: a crafting table, a furnace, a chest, an enchanting table, and a villager trade screen should each read as visually distinct rather than sharing one flat palette. If one specific screen still looks default while others are themed, that screen is likely being overwritten by a higher pack or by a mod that replaces that GUI with its own textures.
When a screen stays default
Disable other resource packs one at a time to isolate the conflict, and remember that modded containers added by other mods may not have themed textures in this pack at all. Those will fall back to vanilla-style art by design.