Okay, so I had some time to take another look. To clarify, 255 does mean fully saturated but it should correspond to white, not black. Black has an RGB value of (0,0,0).
Based on what I can tell, it seems that the blue channel is easiest to threshold. What you could do is use that to create a mask. To me, it seems that 0 to 120 is a good range to select the objects in the image. Once you have converted that to binary, you will probably want to remove noise. There are a lot of different ways to do this. However, your images are pretty clear so simply using the paintbrush tool to manually fill-in and remove the necessary pixels should be reasonable. If you really want an automated way, you can use
Process > Binary > Close. You will probably want to try a few different settings (
Process > Binary > Options...) to see which gives you the best results. I usually set
Count to 4. Based on a quick test, you are probably looking at somewhere between 5 and 20
Iterations to fill the missing pixels. There are a few ways to apply the mask. The simplest is to run
Edit > Invert and then, for each channel,
Process > Image Calculator.... Set the original channel as the first image and the mask as the second. Select
subtract as the operation. This should produce an image with everything but the objects set to 0 (i.e. complete absence of the color associated with the channel). Below is an example resulting image based on the red channel.