Why is BFP screen yellow? But BFN white?

[MrsSam88]

I know this may seem like a very silly question. I was just about to throw my tests away and I noticed the BFP I got at 9dpo has a yellow screen now, I then got a BFN after 4 hour hold that screen is still white. I got another BFN yesterday morning at 10dpo also still white.
Did I do something wrong on my first test giving me a false positive ?

It probably comes from more concentrated urine, FMU would give you a BFP when more diluted urine maybe wouldn't have enough hcg to show up. Up