Hi I have two Acer monitors in dual monitor set up.
X213W and X223W. I can see a clear difference in grayscale / contrast, where one of the monitor is much brighter and clearer and the other is a bit yellowy. I have set the two in exactly same settings on the monitor set up.
Is it just because the other one is defective? Or could you change this somehow?
LCD monitors of the same model using the same exact LCD panel can have some variations. There's always some variations that occur when something something is mass produced.
Monitors of different models are more likely to have differences because there are many manufacturers of the LCD panel. Each panel manufacturer may have slight variations of the manufacturing process itself and different basic materials, as well as different quality control standards.
Using the same settings for the same brand / model doesn't always work. Same settings on different brands / models will typically not work. You will need to tweak each monitor separately. Work on one first until you like it, then play around with the other to come as close as possible. This can take minutes or it can take hours depending on how much tweaking you want to do.