formula/macro to colour blank cell in a range

Try this

Sub Test()
    Dim rng As Range
    Dim cell As Object
    
    Set rng = Range("C2:C" & Range("C" & Rows.Count).End(xlUp).Row)
    For Each cell In rng
        If cell = "" Then
            cell.Interior.ColorIndex = 1
        End If
    Next
End Sub

But some how I suspect that conditional formatting is what you will eventually need

Hope this helps