Function to read cell color and insert Letter?

I would like to write a function where, if the cell at the bottom is red, insert letter H.
I'm not verse with vba excel, hope someone could guide me here.

Function InsertLetter
If cell.Interior.ColorIndex = 3 then
 cell above insert letter H
end_if

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Try this:

Public Function ColorFlag(Cell As Excel.Range) As String
    Application.Volatile
    Select Case Cell.Interior.ColorIndex
        Case 3
            ColorFlag = "H"
        Case 4 'next color test etc.
            ColorFlag = "J"
        Case Else
            ColorFlag = ""
    End Select
End Function

Works great! THanks!