Why is urine sometimes very yellow and sometimes clear?

The yellow colour of urine is caused by several coloured molecules (urochrome, urobilin and uroerythrin) which are produced by the normal breakdown of haemoglobin when red blood cells come to the end of their useful lives.

The variation of the intensity of the colour is simply due to dilution; the more water you drink, the more the colour is diluted.

The color of urine is dependant on the amount of chemicals in it. Drinking a lot of fluid rapidly, will flush the kidneys out into your urine. The first load you eliminate will be colored, the next will contain less color and so on until you stop drinking and pause for the night.

If you're hydrated enough, the water will dilute the colored toxins in the urine.

Toxins in the body.

Too much Vitamin-C (especially via pills) since any extra is eliminated... (Other vitamins will do it, too)

Not enough non-alcoholic liquids (water - dehydration)

Certain medications

Enough water (or non-alcoholic liquids) and it will ... "clear up". ;-)

There are generally two reasons for it,if you take medicine the chemicals in most of the tablets will turn the color to yellow, other possibility is if you dehydrate if you did not drink enough water you would experience yellow urine with bit of irritation