Try this approach - you need to select the paragraphs you want to examine first.
Code:
Sub BoldOfYouSir()
Dim aPar As Paragraph, iColon As Integer, iTab As Integer, aRng As Range, iFirst As Integer
For Each aPar In Selection.Range.Paragraphs
Set aRng = aPar.Range
iColon = InStr(aRng.Text, ":")
iTab = InStr(aRng.Text, vbTab)
If iColon > 0 Then
If iTab = 0 Then
aRng.End = aRng.Start + iColon
Else
iFirst = IIf(iColon < iTab, iColon, iTab)
aRng.Collapse Direction:=wdCollapseStart
aRng.End = aRng.Start + iFirst
End If
aRng.Font.Bold = True
ElseIf iTab > 0 Then
aRng.End = aRng.Start + iTab
aRng.Font.Bold = True
End If
Next aPar
End Sub
Edit: I see you added sample files and the list elements are hard coded. The code I provided ignores automatic numbering but treats those hard coded lists as if they contain a tab (which they do). I don't know how your real source lists are done so this code may or may not be what you wanted.