Definitely clear piping gel. Making your own is super easy. This recipe makes quite a lot but you could probably halve it.
2 envelopes (2 tsp) Knox unflavoured gelatin
2 Tbsp cold water
2 Cups light corn syrup (Karo for those of you in the States)
1. Sprinkle the gelatin over the cold water in a small saucepan and let set for about 5 minutes. Heat on low until the gelatin has become clear/dissolved. -DO NOT BOIL-
2. Add the syrup and heat thouroughly.
3. Cool and store, refrigerated, for up to 2 months.
To colour: Add colouring paste/gel or food colouring drops to get desired colour.
To use: Put icing in bag or plastic squeeze bottle and decorate.
There's a great website with this recipe and lots of tips on using piping gel. It's baking911.com. Just type in piping gel in the search engine and you'll get there.