I'm in my late teens.whenever i ejaculate (masturbate),i've been releasing a yellow/dark yellowish substance along with semen.is this urine?

Seminal vesiculitis?: Maybe an infection in the seminal vesicles (where semen is stored before ejaculation). Or trauma between scrotum and anus, e.g. from off-road biking. An STD (gonorrhea, chlamydia) could do it, but of course only if sex with other people. Most likely not serious, but see a doctor; better safe than sorry. One of your country's excellent sexual health centres would be an excllent choice for care.