here are some probable causes..
This appears to be an older model (over 5 years old) and LCD projectors turn yellow with age. It's just what they do
However, if you are projecting a
computer signal or a component signal, a short in the blue signal of the cable can cast yellow hues over your image. The hue will cover the entire image as if you are viewing the image "through rose (or in this case, lemon) colored glasses" so to speak. That's an easy fix.
Harder fixes: the blue
LCD panel is weaking and/or gone bad.
If you have a round yellowish spot in the middle of your screen, the diffuser has been overheated and is "burnt".