In brief: Not exactly. Blood actually doesn't get thinned.  what we call blood thinners are agents that decrease the ability of the blood to clot, usually used in people whose blood clots excessively.
Taking B-complex vitamins, which include vitamin B12, can cause urine to have a bright yellow or even orange color, but check with your doctor to be sure that's what is going on in your case. B vitamins are water-soluble vitamins, which means that what your body doesn't use is excreted in your urine. Riboflavin (vitamin B2) is especially likely to cause this color change in urine. Several medications can also turn urine a bright yellow or orange color. Changes in urine color may also signal certain health problems.
Also known as urochrome, urobilins are the pigment that gives urine its yellow color. Urine is usually pale yellow, but it can range from clear to a deep amber color in a healthy person. The level of yellow is determined by how hydrated a person is. A person who is very hydrated has more water to release through urinating, and thus has a lesser concentration of urobilin in their urine.