Can you post some screen shots? Are you dragging it under a layer that saturates?
Do both document have the same profiles or are they in different color spaces have different color bit depth. Are they both RGB images. How are you Photoshop Color setting set. How will these two image merge?
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Most likely one of them is untagged (doesn't have an embedded profile).
Pasting an image into another, it will be converted into that image's color space automatically. So even if the profiles are different, it won't change appearance.
Never work with untagged material. There should always be an embedded color profile.
Ah, those comments are helpful, thank you!