Any sign of a colour change on the ketostix and you are in ketosis. As has been said, you are either in or out. The darker the colour, the more concentrated the ketones, and therefore the more dehydrated you are. Therefore, if you test first thing in the morning you may well be right at the top of the scale as you have not had a drink all night and are therefore clearly dehydrated. Test at lunchtime when you have probably already taken on board a couple of litres and the ketones will be diluted, so little or no trace.
Another thing to watch out for as well is testing too soon after having a pack .... The packs contain carbohydrates, and as soon as the body gets easy carbs to use up it will cease burning fat (which it finds hard work) and will leap on the carbs for its energy. You will therefore temporarily not be in ketosis, though you may still measure some ketones. Depending how long you stay out of ketosis, the measurable levels of ketones may drop away completely. DON'T WORRY though, the levels of carbs in the packs is so minimal that you will be back in ketosis pretty quickly, so you won't even notice.
Anything that you put into your body that can be used as fuel and converted to glucose easier than burning fat will always be used by the body in preference. That is why it is so easy to knock yourself out by cheating ......