Just got a new set up: a Mac mini running the latest OS (10.10.5) plugged into a Samsung monitor (model S24D300) by HDMI, and when I wake up the system from sleep mode the screen appears as though the contrast is turned all the way up. The settings on the monitor and within the Mac mini haven't changed, it just looks like the contrast is turned up. I was initially fixing it by unplugging the hdmi cable then plugging it back in. Now I just use the system settings>Accessibility>Display Contrast and turn that slider up then back down and it fixes it. Strange thing is I even took a screen shot early on, then viewed the screenshot after fixing it, and it appeared normal. So apparently the problem is that the correct signal is not making it's way to the monitor upon waking up. Not a big deal, but still would like to get it fixed if possible. I've tried various settings on the monitor itself, but it doesn't make a difference. Could it be the hdmi cable perhaps? Any help is appreciated.