You probably should have one PostProcessVolume with Infinite Extent checked to have control over post-processing because there is default post happening whether you have the volume in your level or not.
That aside, if your texture looks fine, use unlit like Luos suggested and then try disabling the ToneMapper completely with console command:
ShowFlag.Tonemapper 0
And also, you might have some eye adaptation going on so you can also try:
ShowFlag.EyeAdaptation 0
(Default for both variables is 2 if you want to revert to original setting)
If that works for you, you can set it on BeginPlay in your level blueprint like this: