This Theory is a subset of the underdress only theory. The theory goes that sometimes when the Burgundian gown is lifted in paintings you see a different colour at the hem to at the neckline. This difference is because the undergown has an applied boarder or frill on the skirt of a different fabric to the rest of the kirtle. Often the fabric shown at the hem is far richer than that shown at the neckline of the gown so this gives the wearer a way of using a very rich, expensive fabric and showing it off without using much of the fabric. Or perhaps if the hem on a kirtle wore out or became badly stained and was replaced with a different fabric. One quite often sees applied, ruffled bands of the same fabric to a kirtle, so the practice did exist.
1450-1480 Flemish, Bibliothèque de Genève Ms. fr. 64: La fleur des histoires by Jean Mansel. Here the skirt of the gown is lifted up far enough to show the red boarder on the kirtle.
1460, EYCK, Barthélemy d’,René d’Anjou, The Book of Tournaments, Manuscript (Ms. français 2695), 386 x 298 mm (folio size), Bibliothèque Nationale, Paris (detail). Here the gown has an applied boarder of a different fabric which suggests the practice of bordering a dress with a different colour was at least known and done.
1475, MINIATURIST, French, Le Chansonnier Cordiforme (Chansonnier de Jean de Montchenu), Bibliothèque Nationale, Paris. This image is small, but you can see the applied brocade boarder on the black gown. Again, showing there was at least a historic practice of applying a broacade boarder to a skirt.