Galaxy Note 9: Why do the screen color appears slightly reddish?

Note 9 uses the Super AMOLED Screen.  With Super AMOLED, the color is expressive, rich and can be more sensitive to subtle color changes. The color accuracy is high also. 

The  adaptive display mode of Note 9 automatically optimize the color range, saturation, and sharpness of your display. As each user may have different preferred colors for super AMOLED,  if you do not satisfy default screen color,  we recommend to adjust full screen color balance of your display to change the appearance of white tones.

On Apps screen, tap SETTINGS > Display > Screen mode > Full screen Color balance.

Then, adjust full screen colors of display to Cool or Warm tone depending on your preference.

Please note color balance can only be adjusted in Adaptive display mode.

Through advanced options, you can also adjust each value of Red, Green, Blue color for full screen color balance and this feature allows to optimize more broad and detailed the color range, saturation and sharpness of display.