Blurry pic (wrong camera settings) of the first attempt with a completed overgown but a totally faked undergown (the mock fitting bodice and a bunch of fabric pinned to her underwear to emulate a skirt -oo-). The real undergown for this set will be white.
The overgown is Bemberg rayon lining and trim from my stash.

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And a completed undergown with Amelia holding the gray velveteen, red silk, and trim for the overgown.
I widened and lengthened the sleeves for this final undergown version.

Yes, that's the same blue rayon in better lighting and proper camera settings! It is fully lined with some off-white net to stave off color transfer and to provide a bit of body.
The overgown will be gray with red lining.
I may have to cartridge pleat the velveteen to get it pleated up enough: the bodice and skirt are both pleated to the waist band.
I scaled down and simplified the old La Mode Bagatelle Artistic Tea Gown sewing pattern. These are all the view A underdress with view B sleeves and the view B overgown.
As soon as I get around to ordering a bit more fabric (scads of yard goods but none in the right color!) I'll be making a third set with a different overgown pattern out of this material if the velveteen works out
I'm not sure about the trim. I may skip the trim and do some smocking instead.

This is slightly lighter weight corduroy.
I'll use dark teal and purple for the lining and undergown - not sure which will be which.