Blend Mode

Blend Mode makes a difference. Here, working with sketchpad.

Below, simple layering (with 40% opacity):

Blend Mode set a Luminosity (same settings):

source: Moon element is a Canadian Broadcasting Corporation image.

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I can layer one whale on top if the other, and nothing happens.

 If I put the top one in multiply mode, the darks are darker and the lights lighter...

 

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http://www.photoshopessentials.com/photo-editing/layer-blend-modes/color-blend-mode/