How to reproduce:
1. Open the “OverlayUIColorSpace.zip“ project
2. Enable HDR in Project Settings > Player > Other Settings > Rendering
3. Open the “SampleScene”
4. Enter Play Mode
5. Select the “Main Camera” in the Hierarchy
6. Assign the “WorldRT“ Render Texture to the Output Texture field in the Inspector > Camera > Output
7. Observe the Game view

Expected result: The UI Colors stay the same
Actual result: The UI Colors become darker

Reproducible in: 2023.3.0b6, 6000.0.25f1
Not reproducible in: 2022.3.51f1, 2023.3.0b5
Could not test in: 2021.3.45f1 (HDR is not supported)

Reproduced on: Windows 11 Pro (23H2)
Not reproduced on: No other environment tested

Notes:
- Could not reproduce on Vulkan Graphics API
- This is reproducible in Edit Mode as well, however, Play Mode provides some stability when assigning the Render Texture (sometimes the Game view turns black upon assigning)
- HDR supported computer is required for the reproduction