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.
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