The Celts; when I set about designing my Celtic army I started by trowelling through hundreds of reference books and pictures from the Internet. Once I had a taste of the style and motifs involved I set about designing some of my own. I have included some of them here, hoping they may help other people in painting up units of the same type.
For the Celtic Patterns I stuck to very organic shapes and lines, almost plant like, the Art nouveau of it's time. Below them can be seen simple tunic patterns and checks. All these designs are just fiction of course but they do give a very Celtic feel to the shield, but most important, they are easy to paint. Painting my own shields I needed designs that were simple and wouldn't take up too much time. mixing and combining elements from these designs can lead to hundreds of variants. Also by changing the colours, even more possibilities can be achieved.
The Germans; For the Germanic shields I went for a more brutal approach than with the Celts. These patterns are bold statements with big thick dark colours and motifs, giving them a more geometric feel with straight lines and shapes. If there are any curves, they are thick and less plant like than those of the Celts. Also the background colours on these shields will be very plain, being just browns and other dark, muted colours.
I hope these designs help in some way. Being Ancient history, designs and motifs have not really survived to this day, giving a huge chance for the imagination to fill in the gaps. Have fun with it is what I say....enjoy.