This gel colour works in sugarpaste and modelling paste, buttercream, royal icing, glazes and cake batters. That means you can easily match your coating colour to your buttercream decorations without hunting for two different colours that happen to coordinate.
Add it to your batter before baking. The manufacturer states the colour is stable during baking, so the shade should stay vibrant even after your bake comes out of the oven. With red shades, it's worth starting with a light-coloured batter, one without cocoa, for instance. Darker ingredients affect the final colour, and red won't come through as vividly as you want if you start too dark.