Hi Julie,
Thanks for posting you question here. Although I am admittedly not familiar with the other listings of this nature you have seen, I do not believe it is permissible to use a Multi Variation listing to upsell related items (or accessories) in the manner you have described and I do not know of a seamless was to make such offerings on eBay.
However, if you do not intend to charge extra for a gift bag in the example you provided, you could potentially create a Multi Variation listing for which your colors are the only variants and add some text to your description indicating that buyers can send you a note during the checkout process if they would like a gift bag.
You can learn more about Multi Variation Listings on the following page of eBay's Help index:
http://pages.ebay.com/help/sel...ting-variations.html
Of course, it may also be a good idea to contact eBay's Customer Support team to see if they have any specific recommendations with respect to the best way to post such items.
-Mike