Mojave Screenshot Dimmed

Hi friends,

I am a late arrival to Mojave. I want to love the new screenshot method (Shift + Command + 5) and all its options, but somehow, I am just not getting it. I select an area to capture, and it looks dimmed. I initially assumed that was some sort of interface feature, but the screen capture also is dimmed! See example below.

Is it something that I am doing? This can’t be the expected behaviour! This is on a 27” late-2015 iMac (my newer MacBook Air can successfully take screenshots this way, I believe).

Thank you!