That's as fast as its going to move. Its checking over 46000 cells. But if you want the end user to know that the program is still running try this code. It will display "Running Macro" in the status bar while the code is running.

Sub REcolorer()
Application.ScreenUpdating = False
    Dim cl As Range
    Dim active_rng As Range
        Set active_rng = Range("A2:MM1200")
        For Each cl In active_rng
        Application.StatusBar = "Running Macro"
            If cl.Interior.Color = RGB(204, 255, 255) Or _
            (cl.Interior.Color = RGB(204, 255, 204)) Or _
            (cl.Interior.Color = RGB(255, 255, 153)) Or _
            (cl.Interior.Color = RGB(153, 204, 255)) Or _
            (cl.Interior.Color = RGB(255, 153, 204)) Or _
            (cl.Interior.Color = RGB(204, 153, 255)) Or _
            (cl.Interior.Color = RGB(255, 204, 153)) Then
                With cl.Interior
                .Pattern = xlGray8
                .PatternThemeColor = xlThemeColorDark1
                .TintAndShade = 0
                .PatternTintAndShade = -0.499984740745262
                End With
            End If
        Next cl
        Application.StatusBar = False
End Sub