Conditionnal formating for more than 3 colors

public Sub applyColor()
Dim WatchRange      As Range
Dim Target          As Range
Dim cellValue       As Integer


    Set WatchRange = Union(Range("A1:A10"), Range("B5:b7")) 'apply format to these range
    
    For Each Target In WatchRange
        If Target <> "" And IsNumeric(Target) Then
            cellValue = Target
            Select Case cellValue
                Case 0 To 10
                    Target.Interior.ColorIndex = 5
                Case 10 To 20
                    Target.Interior.ColorIndex = 10
                Case 21 To 30
                    Target.Interior.ColorIndex = 6
                Case 31 To 40
                  Target.Interior.ColorIndex = 46
                Case 41 To 50
                    Target.Interior.ColorIndex = 45
            End Select
        End If
    Next
End Sub

2. I'd like to attribute the code to the whole datasheet - is this too much for the program? I tried to put in Range ("A1:ZZ1000") and it doesn't work.. is it possible, or do you advice me to go for the range ("A1:BZ1000")?  
Your code at https://ccm.net/forum/affich-686888-conditionnal-formating-for-more-than-3-colors#5 is not making much sense to me  

'This is saying of the selected range where the change is made is more than one cell then exit  
if Target.Cells.Count > 1 Then Exit Sub  

'Why would you want to loop thru all cells in ' WatchRange when all you are doing is changing color on one cell that is defined by Target  
For Each Target In WatchRange 

'This is an "event" routine. this is fired on its own when on the sheet some data is
'is changed. parameter "Target" that is paased to it is the range of cells thats changed
'and which caused the event ti fire. "Target" could be one cell or more than one cell
Private Sub Worksheet_Change(ByVal Target As Excel.Range)

Dim WatchRange As Range
Dim CellVal As Long
    
    'says if "Target" is blank or is not number, get out of routine
    'this will not work if event is fired for for than one cell
    'this call is good for a single cell
    If Target = "" Or Not IsNumeric(Target) Then Exit Sub
    
    'storing "Target" cell value. Again will not work if "Target" cpnsist of
    'more  than one cell
    CellVal = Target
    
    'setting a range
    Set WatchRange = Range("A1:BZ1000")
    
    'two issues here. one use of "Target" here. This target has nothing to
    'do with "Target" that was passed to routine and contained cells whose value
    'have changed. All this line is doing is looping through cells defined in range
    '"WatchRange", one cell at a time.
    For Each Target In WatchRange
    
        'this will work
        If Target <> "" And IsNumeric(Target) Then