Why do some minor wounds ooze a yellow substance?

If the yellow substance is transparent, it's serous fluid -- the fluid that acts as a medium to hold and move blood cells. When wounds heal, part of the process is to form a sterile barrier, but if this barrier is slightly permeable, and the body flushes serum through it in an outwards direction, it can act as a cleaning mechanism. It's a healthy sign. Cloudy or murky yellow fluid -- pus -- is however a sign of infection. It's a slurry of bacteria, dead and alive, and antibodies, also dead and alive. You can help your body along here by applying an antiseptic like betadyne or hydrogen peroxide.