I don't like using RGB values in Excel Cells. Results can be unexpected. This code run on an empty sheet will list all the Excel Colors and their Indices. 
Option Explicit

Sub ListColorIndices()
Dim i As Long

With ActiveSheet
  Range("A1") = " Color "
  Range("B1") = "ColorIndex"
  
  With Range("A1:B1")
    .Font.Bold = True
    .Columns.AutoFit
  End With
  
  For i = 1 To 56
    With Cells(i + 1, "A")
      .Interior.ColorIndex = i
      .Offset(, 1).Value = i
    End With
  Next
End With
End Sub
If Dr. Coz's Conditional Formatting doesn't work for you, let us know.