I use your form as a Iframe in my web site. So I need to change de background color, not the color of the form. If I leave the background color as gray in my web site your form appears with a big square around. So, how can I change it?
Did you try to play with the "Include page background colors" property on the Embed Form Tab in the Distribute Tab?
If that doesn't help can you please email the link to your web page to firstname.lastname@example.org with a reference to this post.
Please take a look at this FAQ on embed form in general: http://forums.adobe.com/docs/DOC-1991
In the Embed options dialog you will have an option to check or uncheck "Include page background colors". This means If this is unchecked then the form will have no background color and your web page color will show behind the embedded form, the actual background color that you see in your standalone form (not in iframe) will not show.
Note: We do not support modifying the form-in-iframe background color to a custom color other than what you have defined for your web page or what you have defined for your form in FormsCentral.
I hope this clarifies your question, if not, please post the link to your web page with the iframe form if possible.