DP and I have been doing it for 4-5 years now. We started a couple of years before we had our boys, but I can certainly tell if my eldest has eaten a food colouring.
I pretty much do the same as @Elijahs Mum . I buy very little packaged or pre-made food. I also avoid most household chemicals/cleaners/personal care items. It was a hard adjustment to make at first but now I don't even think about it.
There's a great website I use to look up additive numbers, which uses a traffic light system. It's got tons of great info. Not sure if I'm allowed to mention it or not?