"The Red Effect"

I'd recommend then going to the Help for PrPro, and looking for tutorials on using the Lumetri color workspace.

Go the Color workspace, which has the Lumetri panel on the right. It has several different tabs, with different tools in each tab.

HSL is hue/sat/luminance ... you use the controls at the top to create a mask ... and the controls at the bottom to work on whatever comes through that mask.

For this, you would un-check (to turn off) the qualifier sliders for Saturation and Luminance, and above the top Hue slider, click the red dot ... it will set a red hue "key". If you click to turn on the visibility of the mask, and select color/gray, any area that is going to be passed through the mask set by the three qualifying sliders will show as its normal color, all things that won't be affected are grayed out. At this point, you should see some red things and nothing else.

You can "feather" the mask by adjusting a slider control that pulls out triangle-like things on each end of the slider, and width control to widen or narrow the main area of the slider control.

You can also click the option to 'reverse' which means that the mask passes everything but that which you selected with the qualifier sliders. For your wish, select the reds with the qualifier, reverse the mask, and then below use the Saturation control to remove all color other than the reds selected.

Neil