All I know is that the colored impurities are soluble in water as sodium salts.
so what type of compound does this suggest these impurities are?
It's tough to provide an answer when I don't know the question.
What impurities? What compounds?
But in the absence of all of that I would think you're looking at V, Fe, Mn, Ti, Cr, Cu, Co, Ni salts. Most of those compounds are colored and I think all of the Na salts are soluble.