I dont know how but I came across this in the app store: "Model Colors"
Had a google search and according to this website its iOS only but those of you with android, have a search around just in case
You're able to keep a list of what paints you have and make a shopping list of what you need, ready for next time you go to model shops. Looks like you can add kits as well
So for anyone interested have a nosey and let me know what you think