After strenous excercise your body might be dehydrated which makes the blood water level high and hence the urine is concentrated(yellow).

On the contrary, drinking loads of water increases blood water level and dilutes the urine(colorless).

The water in urine is controlled by a hormone called Anti-Diuretic Hormone or Vasopressin. The amount of ADH in our blood is inversely proportional to the water in our urine.