Ketosis

The test strips measure the amount excess Ketones your body is expelling in urine. So the darker the color, the more Ketones and (theoretically) higher degree of Ketosis you are in.

However... urine test strips are highly inaccurate and are pretty much worthless after the first one or two weeks on a Keto diet. Once you are Keto adapted your body will start using those Ketones and less will show up in your urine even though you ARE in Ketosis.  
If you feel you must track your Ketones, get a blood meter or a quality breath meter (not the cheap junk ones). But seriously, you don’t really need them as your body, tape measure, and bathroom scale will tell you of your progress.

I fully agree with  Hutch . The breath meters or blood ketone strips are the most accurate way to measure the concentration of ketones in you blood. Urine strips can vary to some degree overtime as you get keto-adapted.

Each method measures a different ketone.

Blood    - β-hydroxybutyrate (technically not a ketone from a chemists point of view)
Breath - acetone
Urine    - acetoacetate

Cut and paste the text below below into your browser. It links to a good article about the different testing methods:

www.dietdoctor.com/low-carb/keto/ketone-testing-three-ways

I'm not a fan of urine strips. In my experience, you show more ketones when there's less urine in your bladder. It's like all the urine in there dilutes the ketones so picking them up on the stick is more difficult and the readings vary. Having about 200cc in the bladder upon urination, I was getting pretty dark red. Drink a lot of water a few hours later, and the thing was nearly pink.

Hutch In my personal experience, the amount of urine in the bladder when the test strip is used also matters. Large volumes of urine dilute ketones and throw the readings off. I was going with a full bladder and had all the symptoms of ketosis like raging dry mouth yet the strip was showing barely darkish pink. I'd go with about 200cc in my bladder and the strip was crimson. IMO test strips are a borderline sham.

Biff Tannen Didn’t know that about volume effecting urine strip test results. But I wholeheartedly agree with your comment about urine strips in general. ;)