In this case ὑγροῦ is not standing by itself, but has an article that goes with it:
τοῦ τὴν βαφὴν πεποιηκότος ὑγροῦ
Because of the word order, it makes the most sense to read ὑγροῦ as governing the participle πεποιηκότος and its object, which are sandwiched between article and noun.
The translation you propose would require that τοῦ τὴν βαφὴν πεποιηκότος is in predicative position (agreeing with ἐκκαυθέντος), without an attributive article, which seems less natural to me. Also, πεποιηκότος is active -- it makes more sense for the water to be giving depth [of color] than the ashes to be doing so.
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)