One of the first things I hear from folks new to long exposure work is a bright "magenta" cast they sometimes see towards the middle of their images. More often than not the culprit is "light leak." When going through your checklist for setting up your long exposure shot, make sure (if you're camera has one) to close the viewfinder curtain. This will block any light coming in through the back of your camera body. Of course not every camera has this feature, so a simple solution is to carry a bit of gaffer's tape with you to cover up your viewfinder. You can store a couple strips on your tripod legs, or even on lens hoods or camera grips. Unlike duct tape gaffer's tape won't leave behind any residue.
LIGHT purchases their gaffers tape in bulk here at Uline.com