If a woman does ghusl after her menses because she sees clean discharge on her clothes, but the next day sees her discharge is a little darker than usual, should she repeat ghusl?

No. After the clear discharge is seen, any darker discharges (brown, yellow, etc.) occurring are not considered menses.

Narrated Umm `Atiyah (radhiyallahu anha): “After we were pure, we did not consider the yellow or muddy discharge to be anything (i.e. of the menses blood).” [1]

Reference: 

[1] Reported by Al-Bukhari , Hadith 326 and Abu Dawud, Hadith  and “the wording is of Abu Dawud”- Bulugh Maram, pp. 143