Cheers for that input - I think this sentiment can be added to just about every thread on this forum lol.
Just an update for those that do care - I returned to the store I got the white iPhone from this morning. The apple geniuses were really helpful - far more agreeable than I had anticipated.
I showed them the difference between the two phones and the 2 guys that were looking at it actually seemed more taken back by it than I was. They opened up another brand new white iPhone 4 and that one was even more yellow than the one I brought in!
One of them returned with 6 boxes of new white iPhones, and began to tear them all open lol. I felt kind of guilty given that these were all sealed and sale-ready at a retail price of £612 - but again I should stress the staff were more intrigued by this than I was and did this very much on their own accord.
Anyway, by the 4th one he opened, he said that this one looked better/whiter - and I agreed and took that one.
Overall I am pleased, however, even still the difference between the new replaced, white iPhone 4 I have now and the black iPhone 4 I have had since launch is still visible - as in, the black iPhone blue tint still looks better IMO.
Going by the 2 states of color as set out by the OP in this thread - all of the white ones that I saw that were sealed, I would say, were firmly within the yellow tint variety - the one I picked was simply the least yellow of the bunch, if that makes sense...