So we meet again @jonathanm0… As you've undoubtedly figured out, Photoshop and Affinity Photo treat channels quite differently, at least insofar as manipulating/editing them. In order to do what you want, take the following steps: (1) If you are using the Red channel from a single layer, right click on entry in the Channels panel that contains the name of the layer followed by Red. In other words, if your photo is called "Background" then right click on the "Background Red" entry in the Channels panel. It should be the 4th line from the top. (2) If you have more than one layer – perhaps a Background layer and an adjustment or two – right click on the "Composite Red" entry at the top of the Channels panel. (3) Having right clicked on one of those choices, choose "Create Spare Channel" from the drop-down menu. (4) Make sure that the correct layer is selected in the Layers panel. This will be the one with the Red channel filled but with the empty Green and Blue channels. (5) Right click on the "Spare Channel" you just created at the bottom of the Channels panel. (6) Assuming that the layer you're aiming at is called "Background" choose "Load to Background Green" from the drop down menu. (7) Then, right click that same Spare Channel entry and choose "Load to Background Blue" from the drop down.
You should now have duplicated the Red channel into the Blue and Green channels. The layer should now be a monochrome image (since all 3 channels are the same). You can go ahead and delete the Spare Channel, as it has served its purpose.
Use the Flood Select tool. Uncheck “CONTIGUOUS” and set the Tolerance very, very low (probably as low as the slider will go). Click on the grey you want to select.
