Making the Coat
A local production of "Joseph and the Technical-Colored DreamCoat" was done here a few years ago. The "coat of many colors" was made from old neckties, and it was stunning! Perhaps for art you could give them a coat pattern, then have them use different colored papers or fabric pieces to decorate it with.
Or, for art, divide the children into groups. Give them a different piece of a robe pattern (one group gets a sleeve, another group gets one side of the robe, etc). These pieces could be felt, or other plain fabric. Give children colored pieces of felt, fabric, sequins, etc. Don't tell them what their piece is - just tell them to decorate it however they wish. When they are finished, bring all the pieces together and pin on a bulletin board or attach to the wall. I did this at a workshop a few years ago and it was beautiful (and fun!) This could also be done on a banner.