Yes, I'm afraid it is normal for LCD screens to progressively degrade as they get older. However, are you saying that a brand new LCD is starting off yellow (which would be odd), or is your replacement now a few years old as well and starting to go yellow? To improve things, you could try altering the Colorsync profile to shift the hue back towards the blue end of the spectrum. Depending on the OS you are using you will either change it in the Colorsync preferences or in the Display preferences. If you are on OS X, I would use SuperCal (serach macupdate.com or versiontracker.com for it) as it is easier to use than the built-in tools in the Displays preferences.