White balance gains on iOS not independent?

Question

I am attempting to manually control the white balance via iOS 8.2 on an iPhone 5S. I am using setWhiteBalanceModeLockedWithDeviceWhiteBalanceGains.

I have also set the focus and exposure and ISO to manual, and then I am stepping the gain on the red channel from 1.0 to 3.0 while keeping green and blue gains set to 1.0, taking an image of the same (constant) scene at each white balance gains setting. I plot a single row from the image (640 pixels wide) for the red channel (top plot) green (middle) and blue (bottom):

I would expect to see only the red channel change, and yet I actually see a change on all three channels. On green and blue the difference is much smaller, but obviously noticeable.

Am I doing something wrong, or am I misunderstanding how the white balance gains work?

Answer 1

I believe it is right. Let's say that the initial value of red is 255, green 128 and blue 64. If you would apply red 2 green and blue 1 it would be red 255, green 64 and blue 32 due to 0-255 limits. This is the only way to double red channel in this case.