Shield decorations were not divided along Christian/pagan lines at this time. As I wrote earlier, groups of men might have painted their shields all at the same time or in the same colours to express their group identity, and perhaps to benefit from only having to mix up one or two pots of paint. The Gokstad ship included 32 shields painted black and yellow. Perhaps ship crews could have had shields all in the same colour or featuring the same design, and you might decide to adopt this approach for each army, or each group/unit in each army.
There is one example from the sagas of a skald rewarded with a shield that is richly decorated with mythological motifs, so perhaps some shields were more ornately decorated than others. Alternatively, it could be a medieval reimagining of what Viking Age shields looked like based on medieval ideas of heraldry. It’s difficult to know because we just don’t have the examples of surviving painted shields, other than Gokstad.