how do you recolor an image with transparent whites

[dow]

​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.

[Mark Ingram]

Moving to Questions forum.

[toltec]

A bit confused, "I need to recolor an image, in affinity photo, and use the image as a mask."

 

Do you mean something like this ?

 

 

That is selecting the background (blue) and masking the background to reveal the field. Obviously the field could have been a solid colour or any image.

 

I selected the horse with the selection tool, refined the edges to improve them and output to a selection. I then put that above the field.

 

Actually, that's not quite true. It was easier to select the background, then Invert the selection because that was easier than selecting the horse as it was all one colour, unlike the horse.

 

I could have selected the background and erased it to reveal the field but the results would not have been as good.

[Alfred]

I could have selected the background and erased it to reveal the field but the results would not have been as good.

 

It would also not have been as flexible as using a mask. :)