Hi there,
Can you get rid of your merged cells? I highly recommend it. You could try this in your worksheet code pane ...
<font face=Courier New><SPAN style="color:#00007F">Option</SPAN> <SPAN style="color:#00007F">Explicit</SPAN>
<SPAN style="color:#00007F">Option</SPAN> <SPAN style="color:#00007F">Compare</SPAN> <SPAN style="color:#00007F">Text</SPAN>
<SPAN style="color:#00007F">Private</SPAN> <SPAN style="color:#00007F">Sub</SPAN> Worksheet_Change(<SPAN style="color:#00007F">ByVal</SPAN> Target <SPAN style="color:#00007F">As</SPAN> Range)
<SPAN style="color:#00007F">Dim</SPAN> Prompt <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN>, Title <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN>
<SPAN style="color:#00007F">If</SPAN> Target.Cells.Count > 1 <SPAN style="color:#00007F">Then</SPAN> <SPAN style="color:#00007F">Exit</SPAN> <SPAN style="color:#00007F">Sub</SPAN>
<SPAN style="color:#00007F">If</SPAN> Target.Value = "Rib Roof" <SPAN style="color:#00007F">Then</SPAN>
Prompt = "Do you wish to reveres roll the colour?"
Title = "Reverse Colour?"
<SPAN style="color:#00007F">If</SPAN> MsgBox(Prompt, vbYesNo, Title) = vbYes <SPAN style="color:#00007F">Then</SPAN>
<SPAN style="color:#007F00">'... code here</SPAN>
<SPAN style="color:#007F00">'example:</SPAN>
MsgBox "Action performed on " & Target.Address & "."
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN></FONT>
Right click sheet tab name, select View Code, paste on right. Alt + Q will return to Excel and this will fire when you change/enter a cell value. It will exit (not fire) if you have more than one cell selected (does not pertain to merged cells).
HTH