What’s this dark coloring?

No worries so late in the game, good sign that there is efficent metabolism happening, a combo of genetics and environment. Nitrogen is required by the roots to convert sugar into energy, if there is no Nitrogen in the medium the plant can dump its nitrates which are no longer required by the plant and "dumps" it in the soil to feed Micros. In a perfect world you want to have as much nitrates out your plant as possible come harvest so its a good sign. Chlorophylls main purpose is photosynthesis. Anthocyanins main purpose is defense.

Its most likely an access Nitrogen.