I have a Nikon D3100, and whatever exposure levels I try all the images come out in black and white if I'm using manual or any other mode except for the auto modes. What could be causing this?
You have almost certainly set a monochrome "picture control". This is probably showing as an "MC" towards the lower-right of the LCD - if it is, adjust this back to SD ("standard"). (That's for a camera running in English; if you're in a different language, the abbreviations may be different, but the principle remains the same).