Why does urine turn yellow when dehydrated?!

Yellow does mean toxins in your P, what is happening is the toxins are not diluted with water etc, so your kidneys are working "overtime" to detox your blood. Drink more or it can eventually lead to kidney damage/failure.

When your urine is pale, it means you have excess water in your body and your body is getting rid of it through pee.
When your urine is yellow, you are dehydrated because your body has used up its excess water and hasn't got any to spare in your urine.

really . . you certainly are into random questions. So this is what keeps you awake pondering?

Here you go . . I typed "why is urine yellow" in my browser . . it's really not all that hard to do.

http://en.wikipedia.org/wiki/Urine

When you drink alot of water , urine hardly has any colour but when it's concentrated with no water, that's the original yucky colour of urine

Because it lacks the nutrients in water that it needs to get daily.