Is ghusl obligatory for someone who felt emission while between wakefulness and sleep, then saw a colorless spot on his underwear, even though he did not have a dream?

The spot you notice after waking up from sleep, if you are ignorant of its reality, then if you wish, you may consider it maniyy (seminal fluid), and thus you perform ghusl (ritual bath) and repeat the prayers you performed before the ghusl. And if you wish, you may consider it madhiy (prostatic/pre-seminal fluid), and your prayer will be valid if you had washed the impurity of the madhiy, performed wudu' (ablution), and prayed with a clean garment. This is the detailed view of the Shafi'is. If you considered the discharge madhiy but did not wash the impurity out of ignorance, then there is no need for you to repeat (the prayers), according to the more correct of the two scholarly opinions.