Below is a post I saved from another message board.  It was from a woman
who worked for the company who made Ketostix.

Tara

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"The color of the stick shows how concentrated the ketones are in your
urine. So, if you drink lots of water, the stick will be light, and if
you don't drink, it will be dark."
____
This is not the way ketosis works, but is a myth that continues to be
perpetuated because it logically sounds like it should be true. Ketones
are only present in the urine if there are excess ketones in the
bloodstream, and then these excess ketones "spill over" into the urine.
This can happen at various times during the day. Your level of blood
ketosis fluctuates all day long depending on what you eat, and there may
or may not be an excess amount to spill into the urine at any given
time. The amount of ketones is not static in either the bloodstream or
the urine, and their presence in the urine is simply excess that spill
over once the blood ketosis level reaches a certain level. A dark
reading indicates nothing more than the fact that the bloodstream
happened to contain a lot of ketones at that moment, and spilled a lot
of them into the urine at that moment. This is why even if you read
negative at certain times during the day it doesn't mean you are out of
ketosis, it just means that your bloodstream isn't making excess ketones
at that moment. You don't go in and out of ketosis all day depending on
your stick reading, even though you can test positive at one point, and
ten minutes later be negative, or vice versa.

It's also important to know this -- even though the stick is dark and
reads "large," this does not indicate a "large" amount of ketones for
our healthy kidneys to handle. Don't be scare by that "large" label.
Don't forget, these sticks were originally made for diabetics, not
dieters, and a "large" reading to them indicates something completely
different than it does for us dieters. In general, these stix are also
meant for screening purposes and not to make any definitive diagnoses.
If a stick turns up a positive reading of any kind, it's a signal for a
doctor to do further testing.

Drink lots of water, yes, as this diet is a diuretic and you need to
replace fluids...but don't do it simply to change your ketostix color
from dark purple.

--Nancy Eaton (who formerly was a clinical consultant for Ames, which is
now Bayer, who makes Ketostix and many other kinds of diagnostic test
strips and equipment)