Lioness....Shelly is, in fact, correct. Dehydrated people will show ketones in their urine and it is one possibility for why the strips may be purple. I don't doubt that your doctor told you what he did or question his credentials in any way, but doctors for the most part don't test urine. The nurses or lab techs do that. It would be better to ask one of the nurses what those test strips show when a patient is dehydrated. While you're correct in stating that the amount of water that you drink doesn't affect ketosis itself, it does affect the concentration of the urine and how concentrated those ketones will be in the urine. The test strips measure the concentration of the ketones in the urine, so if the urine is more concentrated, the color of the sticks will be darker. It will also be darker if the amount of ketones even in diluted urine is great. The ketone sticks are not always an accurate way of telling whether or not you are in ketosis as they measure fat burned. It could be fat burned through digestion or it could be fat burned through burning body fat. Ketone test strips also only measure one type of ketone and the body actually produces several kinds, so you can be burning body fat and not show a postive result on the ketone test strips.
The strips are only one tool and one that many find too inaccurate to rely on to tell if the plan is working as it should. Taking measurements with a tape measure and weighing periodically as well as how your clothes are fitting are much better ways to gauge success with low carb (can you tell I think those test strips are pretty much a waste of money?).