Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 470/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 391.35
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-14 18:55, hash: rBf7829787da5c Tested first on the night of 2020-09-11 with the same results, not sure of the hash, but it was after the DrawManager merge.
Worked: presumably before rBd6525e8d133b: Use DrawManager for Image/UV Editor
Short description of error
2.91 Image Editor Quick Smoke (Fire) Pure Emission is displayed as (1.0, 1.0, 1.0) even when the actual values are 5 orders of magnitude lower.
Exact steps for others to reproduce the error
Render set to Cycles.
Add Quick Smoke to the default cube.
Switch it to Fire and Smoke
Set cache type to All
Go to a frame like 100
Using the attached .blend you just have to press Bake All and then F12 to render.