This should work:

Sub GreenLetter()
Dim s As String
Dim c As Range
Dim i As Long


s = InputBox("Which letter to greenden?")
LL = Len(s)

If s Like "[!A-Za-z]" Then
MsgBox ("Must specify a LETTER")
Exit Sub
End If


For Each c In Selection
With c
If .HasFormula Then .Value = .Text
For i = 1 To Len(.Text)
If LCase(Mid(.Text, i, LL)) = LCase(s) Then
.Characters(i, LL).Font.Color = RGB(30, 255, 15)
Selection.Interior.ColorIndex =
[A1].Interior.ColorIndex
End If
Next i
End With
Next c
End Sub