AFAIK, it's not possible.

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Doug Steele, Microsoft Access MVP
http://I.Am/DougSteele
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>I have a combo box that is quite long in depth (large no of entries) and
> makes for a confusing list of entries. It would be easier to read if the
> lines were alternating colours.
>
> Is it possible to create alternate line colors for drop down combo boxes?
> The combo box is based on ever changing query data. I just want