The U2515H I tried was supposed to have true 8 bit. That's what everyone, including Dell Hardware support, said last year. I recently searched for the model and now it turns out it may have 8 bit + FRC. The FRC is probably active all the time. I couldn't even look at the white monitor test screen with no cable attached.
In the future I'll only buy monitors that are advertised as true 8 bit explicitly without FRC, on top of that no PWM but direct current and preferably have a VA panel. Because those panels mostly seem not to have FRC.