To really confirm this is a software issue, you should take two phones: one with a yellow tint, one with a blue, apply the same tweak to both and see if they still look different. Something tells me this is a hardware issue, but I'd love to be proven wrong.
The app lets you change the screen to any color you want. So if I can take away yellow, I can add yellow. Not sure what doing it on a whiter screen would prove.
I don't know if it's in fact software. I do know the tweaks works if you want to control the warmth and coolness of the screen.