Please see my code below. What is happening here is when the month changes
to Aug for example certain background colors which were colored in Jul will
now be colored in Aug. Everything works except Now that we are in the month
of Aug, Jul background colors are still colored and I now do not want those
cells to have color anymore. The only coloring should be in current month,
mmm. Can you help fix my code. Very new to this. Thank you.

' ALSO, HOW DO I MAKE THIS CODE ACTIVATE WHEN I OPEN THE WORKBOOK?
'HOW DO I CHANGE THIS CODE SO IT WORKS FOR SPECIFIC SHEETS (I.E. SHEET1,
SHEET2, ETC.) BUT NOT ALL SHEETS
'WHEN I OPEN THE WORKBOOK?
Sub CFormat()

Dim rng As Range, cell As Range
Dim ncol As Integer, lrow As Long
Dim pcnt As Double, divisor As Double

'Sheet1 is sheet name
ThisWorkbook.Worksheets("Sheet1").Activate

' Find column for current Month (add 5 to start in colum F onwards)
' Define "CurMonth" and "HdrMonths" in spreadsheet
ncol = Application.WorksheetFunction.match(Range("a3"), Range("F3:Q3"), 0) + 5

' Find last row of data in current month column
lrow = Cells(Rows.Count, 1).End(xlUp).Row

Range("F9:Q500").Select
'Clears the colours
'Selection.Interior.ColorIndex = xlNone 'THIS STATEMENT DELETES ALL CELL
BACKGROUND COLOR WHICH
'IS NOT WHAT I NEED...I NEED SOMETHING LIKE THE FOLLOWING..

Select Case
Case Is = Interior.ColorIndex = 4
Selection.Interior.ColorIndex = xlNone
Case Is = Interior.ColorIndex = 35
Selection.Interior.ColorIndex = xlNone
Case Is = Interior.ColorIndex = 36
Selection.Interior.ColorIndex = xlNone
Case Is = Interior.ColorIndex = 7
Selection.Interior.ColorIndex = xlNone
Case Is = Interior.ColorIndex = 54
Selection.Interior.ColorIndex = xlNone
End Select

' Set range to cells for current month (WHICH IS CELL A3) starting row 20
Set rng = Range(Cells(20, ncol), Cells(lrow, ncol))

' Set Divisor for current month
divisor = Cells(5, ncol)

' Loop through all cells in range
For Each cell In rng

' Check length of cell in column A
If Len(cell.Offset(0, -(ncol - 1))) = 4 Then
' Calculate perecentage
If Application.IsNumber(cell) Then ' Is this cell a number ?

' Calculate percentage
pcnt = (cell / divisor) * 100
cell.Select
' Set colorindex based on percentage
Select Case pcnt
Case Is 100
Selection.Interior.ColorIndex = 4
Case Is = 90
Selection.Interior.ColorIndex = 35
Case Is = 80
Selection.Interior.ColorIndex = 36
Case Is = 70
Selection.Interior.ColorIndex = 7
Case Is = 1
Selection.Interior.ColorIndex = 54
Case Else
Selection.Interior.ColorIndex = 3
End Select

' This colors the blank empty cells
Else
cell.Select
Selection.Interior.ColorIndex = 3

End If
End If
Next cell

End Sub