I have 2 layers in a photoshop document, both with black tonal imagery. I wish to apply 2 different colours to these black images, using the colour overlay effect. I am using 'screen' blend mode so that the colour only applies to the black image. Which seems to work when only one layer is visible. When both are turned on, the colour overlay on the top layer is tinting the white background of that layer, and therefore changing the colour of the image underneath, as I'm using the 'multiply' layer blend mode. If I cut a 'hole' in the white background I can see the true colour of the image underneath. How do I make the colour overlay just apply to the black pixels, and not to the white background? Thanks.