If I want to colour the cake I use a colour paste which I get on baking shops on eBay . I find that liquid colouring can affect the texture as you have to add a lot to get vibrancy . A paste comes in a tiny pot and is so concentrated you have to add it in tiny amounts on the end of a cocktail stick
If you want really vibrant colours you need to use trex instead of butter. It's white so it makes for a much whiter mix which takes the colour better. Adding a lot of liquid food colouring into a sponge can make it rather heavy. The gels are better as much more concentrated. You can also use trex for the butter cream which is great to get it white.