Could be your white balance. Make sure it is set to auto - if it is set to daylight and you're shooting inside, tungsten light will make things yellow (and fluoro lights will make things green). If things are coming out dark, try adding flash or, if you are already, raise your ISO a bit.
agree with jinx. it is most likely your white balance if your images are yellow. do you have photo editing software? photoshop elements can be purchased for under $100 and will remove any color casts on your photo with a click of a button. it will also help lighten some of the darker photos.
In my experience (which, granted, isn't that extensive) you can probably change the white balance to a custom setting based on where you're shooting, and that should solve the problem.
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