Looking for a bright green dessert sauce for Christmas plate design
ChefTalk.com Top Picks
Of course you can always use green food coloring. But as Pete said, we don't know what you're putting it with.
Then you could make chlorophyll to color the sauce. There's a recipe in the Roux Brothers "New Classic Cuisine" They don't say anything about using it in desserts but I guess it depends.
You might also be able to melt some kind of green candies.
Depending on the other elements of the dish (natch) I would first look at herbs.
Mint is a logical dessert flavour (maybe too much so). If you can get your hands on some fresh Holy Basil that would be a winner. It has an almost tropical fruit quality.
Either way green herbs can easily be blended into something like an angaise. Or just sheared into a syrup, hit with some agar agar and made into a fluid gel.
Think about green beverages (Chartreuse?) that can be turned into a sauce.
Heck, its a Christmas thing, right? A puree of those green "fruit cake" cherries might do the trick, and be seasonally applicable!