Sub b()

Cells(1, 1).Interior.Color = RGB(191, 191, 191) 'красим ячейку
Cells(1, 2) = color_rgb(Cells(1, 1).Interior.Color) 'какой цвет RGB

End Sub

Function color_rgb(ByVal i As Double) As String
Dim r, g, b As Double
Dim exit_for As Boolean
exit_for = 0

For r = 0 To 255
    For g = 0 To 255
        For b = 0 To 255
            If (r + (g * 256) + (b * 65536)) = i Then
                color_rgb = "RGB " & r & " " & g & " " & b
                exit_for = 1
                Exit For
            End If
            
        Next
        If exit_for Then Exit For
    Next
    If exit_for Then Exit For
Next

End Function

For r = 0 To 255
    For g = 0 To 255
        For b = 0 To 255
            If (r + (g * 256) + (b * 65536)) = i Then

r = i mod 256
i = i \ 256
g = i mod 256
b = i \ 256

Sub b()
  Cells(1, 1).Interior.Color = RGB(0, 0, 1) 
  Cells(1, 2) = color_rgb_new(Cells(1, 1).Interior.Color)
End Sub

Function color_rgb_new(ByVal i As Double) As String
Dim r, g, b As Double
    r = i Mod 256
    i = i \ 256
    g = i Mod 256
    b = i \ 256
color_rgb_new = "RGB " & r & " " & g & " " & b
End Function