Color Coding Stops After I get to # 65,282

[p45cal]

The worksheets cells are limited to 56 colors only. If you change the color of a cell to a color not in the palette, it will choose the closest available color.

I don't think that's been the case since around Excel 2003.

If you alter your code to:

For ?? = 1 To 255 Step 7

in the 3 places you have such lines, you should get through the whole range (I doubt you'll be able to tell the difference between two adjacent colours).
Best to start with a virgin workbook each time.

This snippet does something similar:

Sub blah()
Stepsize = 7
Application.ScreenUpdating = False
l = 10
For r = 1 To 255 Step Stepsize
  For g = 1 To 255 Step Stepsize
    For b = 1 To 255 Step Stepsize
      Range("D" & l).Interior.Color = RGB(r, g, b)
      Range("E" & l) = "'(" & r & "," & g & "," & b & ")"
      l = l + 1
    Next b
  Next g
Next r
Application.ScreenUpdating = True
End Sub