highlight / color change, a particular character in a cell of a worksheet

Unfortunately, using VBA is the only way to solve your task. I have modified the code to use the RGB colours. You can access RGB colours by right clicking a cell, then select Format Cells->Font->More Colors->Custom. 

Sub ChangeIncellCharacterColour()

    Dim rSelectedRange As Range
    Dim vMyCharacter As Variant
    Dim c As Integer
    Dim rCell As Range
    Dim iRed As Integer, iGreen As Integer, iBlue As Integer
   
    On Error GoTo HandleErrors
   
    vMyCharacter = Application.Range("rngMyCharacter")
    iRed = Range("rngRed")
    iGreen = Range("rngGreen")
    iBlue = Range("rngBlue")
   
    Set rSelectedRange = Selection
   
    'Loop all cells in the selected
    For Each rCell In rSelectedRange
       
        'Loop characters in the current cell
        For c = 1 To Len(rCell)
            'Set colour of the character if it's value matches our parameter
            If rCell.Characters(c, Len(vMyCharacter)).Text = vMyCharacter Then
                rCell.Characters(c, Len(vMyCharacter)).Font.Color = RGB(iRed, iGreen, iBlue)
            End If
        Next c
    Next rCell
   
HandleErrors:

End Sub

Please also see attached the modified example file

Hope this helps

Kind regards

Yury