Check your pack order
Minecraft stacks resource packs. A pack higher in the Selected list overrides any pack below it. If another pack also changes 
diamond items, it can hide the pink textures.
1. Open Options > Resource Packs.
2. In the Selected column, hover over Cutesy Diamond Tools and use the up arrow until it sits above any other texture pack.
3.
4. Check your diamond gear again.
If you want your other pack's look for everything else, keep both packs active. Put Cutesy Diamond Tools on top so it only takes over the diamond gear it changes.
Other things to try
- Reload textures: In a world, press F3 + T to force a resource reload without leaving.
- Compatibility warning: If the pack shows a red "made for an older/newer version" warning, you can still try enabling it. Some textures may not show if the files don't match your game version.
- Mixed results: If tools are pink but armor on your body is not, another pack is probably replacing the worn armor textures. Move that pack lower in the list or turn it off.
- Shaders: Shader packs through Iris or OptiFine can change how colors look. They should not undo the reskin. Turn shaders off briefly to compare if the pink looks wrong.