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"

)