ASo after a cake fail trying to make rainbow cake using liquid food colouring, I've invested in an Americolor gel colour kit. Are these good for colouring cake batters as well as icings etc? Any tips appreciated on how much to use etc.

yes fine for colouring cake batters. gels are much better than liquids as they don't change the batter consistency as much or at all

biggest tip on how much to use is just use a tiny amount to begin with and gradually a little more at a time IF you need to, after mixing in, until you get your desired colour. don't be tempted to add a lot in one go

I use them for pretty much everything. Batters, breads, icings, to make colored sugar/coconut...you name it LOL.  Start off with just a little bit though because it's easier to add more color than to take it away. BTDT more times than I can count