I use Americolor gel food coloring for work. One drop will color a river. But it's not what you want for char sui. Not even the food coloring from the grocery store (the ones with the pointing caps that always leak everywhere).
What you want is the food coloring Chinese restaurants use which is actually a powder. You'll have to find it at an Asian market (or online) and it gives you the exact color you're looking for. Same stuff they use for sweet and sour. And it doesn't leave an awful taste like the liquid colors do.
(edit: this is similar to what you're looking for. A good market will have about 20 brands to choose from)
