Replacement screen poor contrast?

Hey there,

I cracked my iPhone 4 screen recently and ordered a replacement off eBay. It arrived today and I fitted it without any problems (so I thought).

However, when looking at the screen it doesn't seem right. It's got a strange contrast, that when you tilt it or view it from any angle, the screen is 'inverted', like on older computer monitors.

Is this just a case of me buying a dodgy screen? the ebay seller/reviews did seem pretty good though.

Or could it be something I've done whilst assembling?

Cheers!