With Intersect(Range("A1:A" & Cells(Rows.Count, 1).End(3).Row).EntireRow, ActiveSheet.UsedRange)
.FormatConditions.Add Type:=xlExpression, Formula1:="=MOD(SUBTOTAL(3,$A$1:$A1),2)=0"
.FormatConditions(1).Interior.ColorIndex = 37
Above assumes there are no blank cells in column A.
That is what the code does based on column A.
Is there a problem?
If the code is based on a loop, it would probably be quite slow :
Dim rng As Range, lc&, x&, cel As Range
Set rng = Range("A2:A" & Cells(Rows.Count, 1).End(3).Row).SpecialCells(xlCellTypeVisible)
Cells.Interior.ColorIndex = xlNone
lc = Cells(1, Columns.Count).End(xlToLeft).Column
x = 1
For Each cel In rng
If x Mod 2 = 0 Then Range(cel, Cells(cel.Row, lc)).Interior.ColorIndex = 37
x = x + 1