I think what jakuza is suggesting is more what you want. I'm pretty sure I misread what you wanted (I was thinking you wanted to initialize comboboxes in a userform based on certain inputs from another source). Also, I was thinking you were using a combobox from another userform for the initial dropdown list, but thinking back, I'm guessing you were using the data validation method to populate a list on a cell.
As an aside for your interest though, I can at least explain what I was doing:
I thought you were changing the value in cell A1 via another userform, so CELLREF would become the value of whatever is in cell A1 (e.g. "Blue")
COMBOBOX1 is the name of the combobox in your userform (e.g. cmbCarColour)
You can then select different cases (e.g. CASE1 = "Blue", CASE2 = "Red") based on what value CELLREF has taken on.
So for CASE1, you would be populating the combobox with items from a named range (e.g. RANGE1 = BlueCars). The advantage of named ranges is that they can be from any worksheet, so you can create the list on a sheet protected from the user.
Hope that helps, but I do think jakuza covers what you wanted.