I'd say that as long as you test positive that your ok. I've been on and off of ketogenic diets for the past half year. Ketostix are ok when you start off just to be sure you've dieted correctly and maybe say "yay i'm in ketosis". Mind you, you can test negative on a ketostick if you drink too much water, or if you don't have enough water you might test higher on the ketone levels.
Don't see a real point in aiming for the darkest color.