The key word in the phrase "cake decorating gel" is gel, which is short for gelatin. The same stuff that gives jelly its texture and body is what you need to make your own cake decorating gel at home. It is sold in unflavoured packs in the baking aisles of most supermarkets. Unlike icing, decorating gel is sweet, but doesn't add flavour to cakes. This means it works with whatever decorations you are using on your cake. Mixing up a single batch of clear decorating gel allows you to divide it and create whatever colours you need.
Whisk two sachets of unflavored powdered gelatin into the water in a medium saucepan. Heat over a medium heat, just until steam begins to rise from the mixture.
Add the light syrup to the gelatin and water mixture and stir it in with the whisk. Heat until steam rises from the saucepan again and then turn it off. Do not boil.
Divide the clear decorating gel into small bowls; use one bowl for each colour of decorating gel that you want to create.
Mix a drop of liquid food colouring into each bowl of clear decorating gel with a toothpick. Stir until the colour is completely absorbed and evenly distributed with no streaks. Add more liquid food colouring, if necessary, a drop at a time, until you reach your desired shade.
Store any unused decorating gel in airtight containers or plastic bags in your fridge.
You don't need a special piping bag to use your homemade decorating gels. Just put some decorating gel in a sandwich bag, squeeze the air out and squish the gel down into one of the bottom corners of the bag. Push the gel back a little from the very tip of that corner and then cut it off. You now have an instant disposable decorating gel piping bag, suitable for writing words or drawing line art on cakes. Use a new bag for each different colour.
You can store clear decorating gel in airtight containers or bags in your fridge for later colouring, if you like. Bring it to room temperature before adding food colour, so it will be easier to stir the colour in.
Avoid the temptation to add a lot of food colouring at once. Since the decorating gel is clear, it only takes one drop of food colouring to add colour. By adding it a drop at a time and then stirring it in, you are more likely to arrive at exactly the shade you want.