Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce RTX 2070 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.92
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-06-25 21:53, hash: rB62aa103d485f
Worked: (newest version of Blender that worked as expected)
Short description of error
Hair colored by vertex colors rendering as black in Cycles. Vertex Paint preview on the Viewport shows as 100% white.
Exact steps for others to reproduce the error
Add a cube, subdivide it by 3, add hair to it. Now paint some vertex colors. Add a material and add a Vertex Color node. Try to preview it, it shows as black in Cycles. It's also missing from the viewport preview.