Is ghusl required if one feels a very slight wetness, colorless and shapeless, with a sweat-like odor, in the underwear after masturbating?

Masturbation is forbidden, and repentance from it is obligatory. Ghusl (ritual bath) is not required unless semen, which necessitates ghusl, is discharged—this is followed by a weakening of the body. As for madhy (pre-seminal fluid), it necessitates wudu (ablution), istinja' (washing of private parts), and purifying the clothes. If you are unsure whether the discharge is semen or madhy, you have the choice to assign it the ruling of whichever you prefer.