Use more or less depending on how dark you want the colour to be. As the pastes dry your colour will darken slightly so a little goes a long way, which just means you’ll have these pastes last you through a lot of bakes.
For ease try to use a toothpick to dip into the gel paste and then add into your edible medium for a mess free and easier method. This also means you can accordingly create different shades.
With so many colors on offer the choice is endless, get creative!