You have turned on the Highlight Clipping warning. With clipping warnings turned on, highlights with no detail will show as red and shadows with no detail will show as blue.
The [ J ] key will toggle these on and off. Also, the controls for them can be turned on or off individually for shadows and highlights. Those controls are located in the Develop module's Histogram panel. Click on the small triangles above the histogram to turn either Shadow Clipping warning or Highlight Clipping Warning on or off.