Try this: I believe you may be better off using the double IIF:
=IIF(Fields!PicturesbeforeFirstSignoutCount.Value < .80, "Red",
IIF(Fields!PicturesBeforeFirstSignoutCount.Value >= .80 OR Fields!PicturesbeforeFirstSignoutCount.Value < .95 "Yellow",
IIF(Fields!PicturesbeforeFirstSignoutCount.Value >= .95, "LightGreen", "White")))
OR if you want...