Photo colours change according to background

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I am using photoshop 7.01 which works fine for my requirements.

I have, over many years, taken hundreds of product photos, trimmed away the background and pasted them onto new backgrounds with little trouble.  I have just tried to do this with a bag of salt; seee image attached.  When pasted over a background with different colour bands, the respective parts of the bag of salt take on the hue of the background. 

To the left of the bag of salt is another image, processed in exactly the same way, but where the white of the image remains true when crossing over the background colour bands.  It is not just an optical illusion. As far as I can tell the bag of salt photo is not a transparent image it is set to 100% opacity.  On the right is the same bag of salt pasted over a vacant background and showing the correct colours.

Can any one explain this, or what I should do to avoid this when I cut the image out of its background in the original photo.

[gener7 · Accepted solution]

It looks like a Layer Blending mode setting. Are both layers set to "Normal" in your Layers Panel?

[default2hl3b4w6qob4]

Thank you gener7.  Such a simple solution and yes, one of the layers was not set to normal.  problem solved. After more than 30 years of using Photoshop there is still so much to learn!