I've realised 4 layers in one of my compositions is the wrong colour. The layers all have keyframes.
Is there a way to change the colour (I used Illustrator to create all the layers) and it update in my composition? Or do I need to redo the layers and keyframes all over again?
You need to be more specific. Is the color of the layers wrong or is the color of the artwork ON the layers wrong? It might halp if provide a screenshot indicating exactly what is wrong.
You can use the Edit Original features (learn more about it in by using the search help field at the top right corner of AE). This will keep all of your existing animation and everything will stay the same if you only change the color.
Another option would be to use the Fill Color, Change Color or any of the other color correction tools available in AE. Without more workflow details it's hard to say what will be most efficient.