Option 1 (Simple and direct):
> One pen is red and one green, and they are on the art table being used for making Christmas decorations.
Option 2 (Emphasis on the pens):
> The red and green pens are on the art table, being used to make Christmas decorations.
Option 3 (More descriptive):
> A red pen and a green pen sit on the art table, their colorful inks being used to create festive Christmas decorations.
Option 4 (More concise):
> A red and green pen pair are on the art table, making Christmas decorations.
The best option will depend on the specific context and desired tone of your writing.