Frustratingly, we don't know. Everything we know about shields in this period comes from archaeological sites (we don't have any contemporary art showing shield faces), and the fronts of shield, along with any decoration they may have displayed, have never survived.

We do know from archaeological excavations that shields were always covered with leather (probably rawhide), and in one case (Tranmer House) this leather was covered in some sort of preparatory layer for paint (maybe gesso); but, the painting didn't survive. Many shields had metal appliqués (like Sutton Hoo, often less fancy), and some had a pair of large flat circular rivets whose functional purpose isn't clear (so, possibly another form of metallic decoration). But what, if anyting, would have been painted onto their faces is unknown.

In many cases, shields were buried to display their faces toward assembled mourners, and several graves at Mucking had shield boards (noe visible only as a dark soil stain) buried in the grave eithout their metal bosses. To me, that implies that shield faces were decorated: you wanted guests at the funeral to see the shield face and know who your family's friends were by recognizing the design(s) painted on it (even if you wanted to keep and recycle the still useful iron components).

We do have a record of late Roman shield designs, the Notitia Dignitatum. It's very possible that some of these designs would have continued to be used in Britain after 410. One might also wonder whether some of the circular motifs on Style 1 jewelry were also used to decorate shields, but that's just speculation with no hard evidence to support it.

If you want to read further, see Dickinson and Harke's book on early Anglo-Saxon shields, available free on Harke's academia.edu page.