LCD "Background" / OFF Colour (iPhone)

Hello,
I have 6 iOS devices next to me:
(iPod Touch 5g, iPad 1, iPad2, iPhone 4, iPhone 4, iPod Nano 6g)

All screens are turned off and all screens appear in a very dark blue.

Except from one iPhone 4 which has a deep purple coloured screen.

What is the factor for LCD screen influencing the background colour?
And how is it possible to have such difference with the same device model?

Thank you in advance!
xKbtAsked:
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Paul MacDonaldDirector, Information SystemsCommented:
You're seeing the reflection of light off the LCD panel itself.  The variation in color is likely due to the materials used in the manufacture of the display and/or the resolution of the display.

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