as far as i know, no 2 amoled screens are made the same, but here's the bad news, amoleds are made blue-er than usual for a special reason, the organic substance inside the amoled will degrade with usage , so if yours is yellower it means your amoled screen is more degraded than your friend's, which also explains why his is brighter, but there shouldnt be such a huge difference, if really obvious, go and replace it

No this is incorrect, it's just two different screen that existed in SGS2. And it's also ridiculous to make it display blue stronger as this will only speed up the degration. 

888kb24, you can see one thing, it's true that yours is slightly yellowish that it should, but your fren one is also at the same time slightly blueish than it should(which is what most of us own). And you can compare a 2nd thing. With your fren unit, when u turn the unit even ever so slightly off angle, the whole screen will get even bluer, a strong blue tint will cover the whole screen while viewing off a straight angle. While with your yellowish screen, you should be able to off angle your unit quite some degree to get the screen to turn blue slightly only. I'm still not 100% sure so u might be able to test it out.

If it's like what I said, then it's up to u whether u want a blueish screen with serious viewing angle issue, or a slightly yellowish screen with pretty much no viewing angle issue.  The sad thing is none are perfect though

yea mine also 5 degrees but can only notice if you're in a pure white background if not it's hard to notice unless you tilt alot la, budden the viewing angle