You'll get it from your local Halfords or any decent motor factor. Something like Bluecol will do the trick, but there's a variety of makes around, and they're all the same sort of stuff.
I doubt if it'll go brown with a HG failure, mainly because it starts off blue or green TBH there are so many other ways of detecting things like this that IMO its not worth worrying about.
In reality, Toyota 4life is just ethylene glycol pre mixed with distilled water, but sold with an air of mystery surrounding it, so people pay the exhorbitant price for it.
If you want a second opinion, talk to CW, I'm pretty sure he'll say the same.