Got a message alerting me to this. Apologies for the error. I'll post a correction in the pinned comment. Here I was hoping that the biggest mistake I made was leaving Ambient Occlusion turned on...
Ah, dug around in the settings and found that I had Tonemap turned on in Postprocessing. I don't remember turning that on, but I likely did it a while ago and forgot. I do love crushed colours in my shooters. Thank you for the prompt.