I image it converts the null values to 0 for the comparison and short circuits to Red. Try this, also Switch is much cleaner then nested IIFs:
=Switch(
IsNothing(Fields!GrandTotalBillableRatio.Value), "Silver",
Fields!GrandTotalBillableRatio.Value >= 2.0,"Green",
Fields!GrandTotalBillableRatio.Value >= 1.75, "Yellow",
Fields!GrandTotalBillableRatio.Value >= 0, "Red",
True, "Silver"
)