How can I remove one element in a field without removing all values and replacing them?

I want to remove a element from a multi-valued field (specifically, the third element).

myfield(0)-"red"
myfield(1)="blue"
myfield(2)="white"
myfield(3)="green"

As with any other task that's more complex than you might like, you can make it easier on yourself by modularizing the code into reusable bits. For example, you could use the following routine:

Sub RemoveElementFromField
(doc As NotesDocument, Byval 
fieldname As String, Byval index As Integer) 
        Dim value As Variant, i As Integer 
        value = doc.GetItemValue(fieldname) 
        If index > 0 And index 
<= Ubound(value) Then 
                If Ubound(value) = 0 Then 
                        doc.
ReplaceItemValue fieldname, "" 
                Else 
                        Redim Preserve 
value(0 To Ubound(value) - 1) 
                        For i = index To Ubound(value)-1 
                                value(i) = value(i+1) 
                        Next 
                        doc.ReplaceItemValue 
fieldname, value 
                End If 
        End If 
End Sub

RemoveValueFromField doc, "Colors", 2

Function ArrayRemove(arr As Variant, Byval ind As Integer) As Variant ' Returns the array with the element at index 'ind' taken out.
...

doc.Colors = ArrayRemove(myfield, 2)

Function SearchAndDestroy(arr As Variant, 
value As Variant, Byval compMethod As 
Integer) 
        Dim pos 
        pos = ArrayGetIndex(arr, value, compMethod) 
        If Isnull(pos) Then 
                SearchAndDestroy = arr 
        Else 
                SearchAndDestroy = ArrayRemove(arr, pos) 
        End If 
End Function