I just noticed that when you DrawMaterial in the HUD it comes out a much darker shade than simply drawing the texture. It even occurs in the content examples, and it is the same on Mac and PC.
I figured out that it was because of the texture compression settings, at least in my case. It somehow got set to TC_EditorIcon instead of TC_Default, which apparently makes Unreal apply a 2.2 gamma correction to the image.
This turns out to be a false lead. What actually fixed it was fiddling with the RGB curve. Once I did that setting the RGB curve back to 1.0 had no effect anymore (another bug?). Using the value .454545 gets pretty close to the original colors.
Hello,
Thank you for your report. We were not able to investigate this on the engine version you reported, but there have been many version changes to UE4 since this question was first posted. With a new version of the Engine comes new fixes and it is possible that this issue has changed or may no longer occur. Due to timetable of when this issue was first posted, we are marking this post as resolved for tracking purposes.
If you are still experiencing the issue you reported in the current engine version, then please respond to this message with additional information and we will investigate as soon as possible. If you are experiencing a similar, but different issue at this time, please submit a new report for it.
Thank you.