Quick answer
Do I need ghusl after spotting?
Ghusl is only obligatory if the spotting was part of ḥayḍ. Light spotting that is not ḥayḍ — for example after the signs of purity, or too brief to count — does not require ghusl; wuḍū is sufficient to pray.
Ghusl becomes obligatory only when spotting was part of ḥayḍ (or nifās). If the spotting occurred within your recognized menstrual window and the cycle then ended with the signs of purity, ghusl al-ḥayḍ is required before you resume prayer.
If the spotting was not ḥayḍ — for example, light brown or yellow discharge after the signs of purity had already appeared, or bleeding too brief to meet the minimum duration of ḥayḍ in your madhhab — then ghusl is not required. You remain in a state of purity and only need wuḍū before each prayer.
Spotting classed as istiḥāḍah (irregular, non-menstrual bleeding) does not require ghusl each time. The woman performs wuḍū at the start of each prayer time and prays, as the Prophet ﷺ instructed Fāṭimah bint Abī Ḥubaysh (raḍiyallāhu ʿanhā).
In short: ask whether the spotting marked the end of a genuine ḥayḍ cycle. If yes, ghusl is obligatory. If the spotting is istiḥāḍah or post-purity discharge, wuḍū before each prayer is sufficient.
This summary draws on the established positions of the four Sunni madhhabs. For your specific situation, consult a qualified scholar.
Go deeper
Ghusl after menstruation: step-by-step