I cut and pasted this information from www.ketoslim.com They are makers of ketone testing strips.

I'm following Induction strictly; why won't my strips turn purple?

Ketones will spill into the urine ONLY when there is more in the blood than is being used as fuel by the body at that particular moment.

You may have exercised or worked a few hours previously, so your muscles would have used up the ketones as fuel, thus there will be no excess. You may have had a lot of liquids to drink, so the urine is more diluted. Perhaps the strips are not fresh, or the lid was not on tight and some moisture from the atmosphere got in.

Some low carbers NEVER show above trace or negative even ... yet they burn fat and lose weight just fine. If you're losing weight, and your clothes are getting looser, you're feeling well and not hungry all the time .. then you are successfully in ketosis. Don't get hung up on the strips; they're just a guide, nothing more.

Will I lose weight faster if the strips show dark purple all the time?

No. Testing in the darkest purple range all the time is usually a sign of dehydration -- the urine is too concentrated. You need to drink more water to dilute it, and keep the kidneys flushed.

The liver will make ketones from body fat, the fat you EAT, and from alcohol --- the ketone strips have no way of distinguishing the source of the ketones. So, if you test every day after dinner, and dinner usually contains a lot of fat, then you may very well test for large amounts of ketones all the time. However this does not indicate that any BODY fat was burned.

The strips only indicate what's happening in the urine. Ketosis happens in the blood and body tissues. If you're showing even a small amount, then you are in ketosis, and fat-burning is taking place. Don't get hung up on the ketone sticks.