[need decompose, compose]

If you add decompose, I'll quit using GIMP for that. I have to use GIMP to decompose an image to CMY or RGB and back. Then I can work with the files in AP but then I'll need to go back to GIMP to compose the layers back to regular images. Decompose/compose is a great feature and ought to be in affinity photo. It should be easy, right?

[how do you recolor an image with transparent whites]

​Below is a snapshot from GIMP. I need to recolor an image, in affinity photo, and use the image as a mask. how do you do that? I'm having trouble making the whites transparent. Is there an alternative to "darken only"? 

 what I want to do is make 3 images to appear like a photo.