How to reproduce:
1. Create a new HDRP project
2. Create any VFX Graph
3. In the VFX Graph Blackboard, create the "Color" property
4. Expand it and click on the value to open the "HDR Color" window
5. Observe the results

Expected results: The picker is functional
Actual results: The picker is not functional and the window is broken

Reproducible with: 6000.3.0b6 (both revisions), 6000.4.0a2
Not reproducible with: 6000.3.0b5
Can't test with: 6000.0.60f1, 6000.2.8f1

Reproducible on: macOS Sequoia 15.6 (M1)
Not reproducible on: No other environment tested

Note:
- Warnings are also logged into the Console:
Unable to find style 'ColorPickerBackground' in skin 'GameSkin' mouseUp
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:224)
- The issue is not always consistent, it seems to be dependant on the cache