Specimens of AATCC L2 and L4 blue wool fabrics were exposed to various combinations of temperature and humidity. Color measurements were taken immediately upon removal from exposure conditions and compared with measurements taken throughout a 24-hour period following exposure. Both L2 and L4 showed color instability following exposure, regardless of temperature and humidity during exposure. The tests indicate that the sensitivity during exposure might be the cause of fabric color instability following exposure. This post-exposure sensitivity limits the precision of lightfastness test results.