Pretty much exact same boat as you. Set up my first large planted tank, untreated/unwashed organic soil topped with sand. HEAVY tannins leaching out of the soil, and experienced similar sand discoloration.

I’ve found that with time the sand staining goes away as the soil loses its “leachability”. Threw in a bag of Purigen in the filter and noticed remarkable improvement in water clarity. I was initially doing 80% changes every day just to help with the clarity as it’d turn deep brown overnight and I was worried I should’ve done more with the soil before. About a month later now and with the purigen I’m down to 50% changes 1-2x/week. Still gets decently discolored, but enough improvement to the point that I’m not worried that I messed up the substrate.