Change font colour

Hi I am stumbling with this so ask for your help please.
My spread sheet has dozen or more columns with cells that contain multiple lines of text.
In one column some of these lines of text are colored green and I would like code to hide the lines that are green by coloring them white when necessary, then un-hide by changing them back to green.
I found and adapted this code but it looks at the entire spread sheet instead of just one column and takes forever before crashing.

Sub colorWhite()
Dim i As Long, c As Range
For Each c In ActiveSheet.UsedRange
    With c
        For i = 1 To Len(.Value)
            If .Characters(i, 1).Font.ColorIndex = 4 Then
                .Characters(i, 1).Font.ColorIndex = 2
               
            End If
        Next i
    End With
Next c
End Sub

Re: Change font colour

Thanks Alan but it appears to only works in cells where all the text is green. Each of my cells has lines of text that are green and lines of text that are black and I only want to change the lines that are green.

Re: Change font colour

Hi Alan. Cheers I sort of worked out adding your code for myself but it takes forever as it looks at each character and there are 1000's of them in the column. This is what I have now.

Sub colorWhite()
    Dim c As Range
    Dim lr As Long
    lr = Range("M" & Rows.Count).End(xlUp).Row
    Dim rng As Range
    Set rng = Range("M1:M" & lr)
    For Each c In rng
        With c
            For i = 1 To Len(.value)
                If .Characters(i, 1).Font.ColorIndex = 50 Then
                    .Characters(i, 1).Font.ColorIndex = 2
                End If
            Next i
        End With
    Next c
End Sub